The whole sirloin?? What size? What shape?
My first thought is to marinate in olive oil, lots of fresh garlic, oregano and rosemary and perhaps a little bit of a whit wine....then add some lemon juice about1 5 minutes before grilling medium rare....but you don't like meat that isn't well done.
Depending ont he age of the animal, a sirloin might be tough roasted well done...might be better in a braise.

Never heard of a sirloin of lamb! I think it should be roasted or grilled....to however well cooked you like. You must have rosemary and garlic......AH but before you decide, check out chefsteps where they do a leg of lamb and serve it with amazing crispy mint topping..... sous vide is probably the best way to go.

If I have a smallish bit of lamb, I like to butterfly it and bbq it.

We always eat it with crunchy roast potatoes and flageolet beans, with a jus.

Sorry, no, I didn't write that well. It's like 1.25 lb. — a hunk, not a whole sirloin.

I grew up eating a lot of fine lamb—and never with rosemary, garlic or mint. I’ve tried them with lamb as an adult and really don’t like them at all. I think they kill the flavor of the meat. Way too strong for lamb. Better for something gamy. Thank-you both for the recommendations. I guess I was looking for something a little less standard.

It's not just for me. I'll be aiming for med-well, which is the classic temperature for lamb. There will be enough that's well done enough for me.

I don't know for sure, because this is from a different source than my usual, but I'm pretty sure it’s proper lamb, not baby mutton. There's no way I'm braising it!

I hadn't been thinking herbs. Maybe tarragon? I think that would be good.

some cinnamon might be nice, not much.

tarragon can quickly over power things to some people ... but that doesn't bother me. love tarragon and potatoes. this sounds tasty ...

I’m jealous, so rare to see more than the few standard cuts here. I’ve been cooking more lamb lately, and would love to get more variety.

I looked up sirloin and see F&W calling it “the best cut you’re not cooking.” They’re using ras el hanout which leans the direction bragu is steering you. I might give that a try.

https://www.foodandwine.com/lifestyle/why-lamb-sirloin-roast-best-cut-youre-not-cooking

With British parents, I grew up eating a lot of lamb.......and mutton. Personally, I cannot imagine a lamb roast without the flavorings of rosemary and garlic and lemon. IMO, they never overwhelm the delicate favor of lamb any more than they do as seasonings for chicken or other poultry! I would season as lindac92 suggested and roast or grill to medium rare. Also IMO, lamb should never be cooked beyond medium rare (mutton completely different) as you will lose all the flavor if overdone.

Dijon mustard is also a popular lamb seasoning. Or you could go a more Middle Eastern/Morrocan route with tagine seasonings however I think that is more appropriate to braised presentation common to mutton or tougher lamb cuts.

I agree with rosemary, garlic and lemon working especially well with lamb.

I often buy a butterflied leg of lamb to grill. I would also grill the cut you have. As gardengal posted, Dijon mustard is a popular with lamb. One of my favorite marinades for butterflied lamb is the following. I also like to use it for grilled pork tenderloin or boneless skinless chicken breasts.

Dijon Mustard Marinade

2 cloves garlic, minced
1/2 tsp. salt
2 T. Dijon mustard
1 T. soy sauce
2 T. lemon juice
1 tsp. dried rosemary or thyme (or 1 T. fresh) - also good with herbs de Provence
1/4 cup olive oil

Combine and pour over lamb, coating completely. Cover and place in fridge for several hours or overnight, turning occasionally. Grill to desired doneness...yes, medium rare!

Yeah, when you're very used to certain strong flavors, you can kind of subtract them in your head and taste around them, including the notorious salt. I can't imagine liking rosemary and garlic on chicken either. But maybe the rosemary in my garden is a lot stronger, and the tarragon a lot more delicate, than what grows in other regions. I can't imagine tarragon overwhelming anything, but I believe Dave when he says he does.

FOAS, I think I have some pre-mixed ras al hanout. It's also pretty strong, but they say half a tsp, which is just a whiff. The F&W preparation is pretty much my starting point, thanks for the link, other than plating for the socially distanced dining room, not crowded around the cutting board. Maybe I should make a sauce instead of looking for a different treatment. As to sourcing, I think we're getting meats originally destined for high end restaurants. California used to have fabulous lamb, but a lot of the rangelands have been paved over. I think this is California, however. Most of the time, the best lamb I've been able to get is New Zealand, and I'd think they'd ship that at least partially broken down, which means only the most popular and profitable cuts. Maybe you can find a butcher near you who carries locally grown, head to tail.

If you are a "purist"....skip any seasonings but for maybe a rub of butter or olive oil before grilling. I grew up on grilled lamb chops with only a sprinkle of salt...very good if the lamb is good.

Yeah, Linda, you're right. It has plenty of fat and doesn't need the butter, i should just cook it plain, as usual. The only reason I was looking for something else to do with it was boredom. I can make a sauce or puree as a condiment. I have a bunch of baby red carrots and some beautiful chard, to go with it.

IC, you're so right about dosage! I like flavor, but most people are trying to hide the lamb flavor, IME, and drown it. I'm not talking about our people, but what I've encountered IRL.

We had lamb last night. Roasted untkl pink with potatoes with garlic, olive oil, red wine, salt and pepper, served with jus and flageolet beans. Simple, easy and good.

I forget to report back. I wasn't feeling great so just cooked the lamb following the pattern in the F&W recipe FOAS linked. I'm not a big believer in rubs or seasoning before cooking, but the sirloin was so small--1.33 lb., where the recipe said 1.25--that I thought most bites would get some crust. Instead of following the recipe, however, I just sprinkled as I browned. I think it was salt, lemon pepper, mustard powder, dry basil and smoked paprika. It may sound funny, but I was licking my fingers after I pulled off the ties (yes, I washed them after) and it was super good. I roasted it in the same little cast iron skillet I browned it in, which may be why it ended up more cooked than they said--I wanted medium well, and held my breath and got perfect medium well, so that worked. :) It was beautiful, tender and tasty.

Thanks to all for the help!