a sirloin tip roast would not be my first choice for roasting medium rare...I leave that for a loin roast or a rib roast.,...It will be a little tough. I would pot roast it.
But if you are determined on a medium roast.....I would say a marinade would work very well....and for more than 30 minutes....over night would be best.
If you sear it to a nice brown, chances are huge it will be over done. I would check temperature with an instant read and test in several different spots.

would roast it at 425...until temp measures 115....cover and let it rest for 20 to 30 minutes....can't tell you how long because I don't know how cold it will be when you put it into the oven nor how many times you will open the door nor how much it weighs.

Thanks, Linda. Yes, sirloin tip is not my impression of a tender piece of meat. When I buy a whole piece I cut it up thin for stir-fries.

This looks pretty lean.

When I want a pot roast I usually buy chuck. It’s usually fatty enough that I know it will be tender.

But I should at least attempt this rare. We can always just re-roast it the next day if that doesn’t work out.

If you cook it rare and slice it very, very thin.....almost shaved it will be great.....great flavour. Perfect for French dip

I haven't cooked on in years, but my first MIL and my own mother did fabulous sirloin top roasts and they were always rare and sliced thinly. Delicious!!

It turned out perfectly! Just the right color inside. It was not the most flavorful piece of beef, but the texture and tenderness was good.

I marinated for about 6 hours in red wine, seasoned, then roasted at 400 degrees, using a leave-in thermometer.

Next time I'll leave it out to warm up to room temperature before roasting. It spent half it's time in the oven just reaching room temperature. But I was out and delayed and didn't get home in time to do that.

It seems like beef flavor varies a lot. Even with sirloin steaks some are flavorful and others are just, well, meat.

Sirloin is one of my favorites, usually it's very flavorful, although not always very tender.

I usually roast a sirloin tip just as you did, at high heat until rare and then very thinly sliced across the grain. It doesn't melt in your mouth, but it's plenty tender enough to suit me and makes a fine sandwich later.

Annie

I did a chuck roast on the weekend. I always cook them well-done. This time I put balsamic vinegar on it, and made a paste out of garlic, olive oil, thyme, rosemary and salt and pepper and spread it over the roast.

Cooked it until 125, but the inside cuts were too rare for us. But thinly sliced it was tender. I put it back in the oven briefly and we'll be making nice beef sandwiches out of it. Either as hot beef sandwiches, or as dipping sandwiches.