No. The topping will get soft and rubbery, then potentially dried and hard.

After the casserole is pretty well heated in the MW, you can add some cheese and zap until it's melted, but it won't toast. Unless it's a fancy microwave with a speed element or a browning element?

I don't know if this would work, but you could test it at home with a small portion--pre-toast the bread crumbs on a sheet in the oven, then warm them up with a dab of butter on the stove in a frying pan. You could then top the gooey cheese topped casserole with the toasted crumbs. A powdered cheese, like finely grated parm, might even melt some in the heat of the crumbs. But they might come out sandy or greasy, and not be worth the effort. I haven't done this, mind you, I just think I might try it.

The easiest and best bet is probably to just make it the first way and maybe add just a little extra cheese to the top when you reheat it, for pretty.

Yeah, that's the way I'm thinking. I am unaccustomed to bringing a dish like this to someone else's home! Everyone should come here!

I love creamed onions and make them often. I think I'm the only one in my family who does :-) I prefer to use jarred onions (different flavor from fresh or frozen) and I just make a well seasoned bechamel cream sauce for them (including a good swig of sherry or brandy). No cheese, no other toppings. They reheat beautifully in the microwave but might need just a splash more milk or cream.

The past few years I've gone through a lot of iterations of trying to cook in someone else's kitchen, doing it here and bringing it, and doing part here and part there. None of it is fun, but I finally have it pretty well figured out which things are best done how.

One dish isn't too bad. Any chance you have a Nesco Roaster? That's essentially and oven--or at least my mother's ancient GE version is, and is great at reheating casseroles. If you do, and the casserole will fit, you could ask your hosts if there's a room away from the kitchen where there's a different electrical circuit (not one that someone's blow dryer will be on!) where you could set it up. If you don't know them well enough to ask that, stick with plan A. :)

I don't think there will be room for a Nesco even if I had one, which I don't.

We don't exchange gifts. ;-)

I have done the precooked buttered crumbs on top of a faux baked casserole....works fine
I toast home made bread crumbs and when dry and browned add butter....enough to make "clumps" like a streusel and stir in some grated parmesan cheese from the box, and refrigerate. Then heat the casserole in the microwave....add the crumbs....and microwave a short time more just to warm the topping.
I do this a lot at Church....but the "casserole" is in a pot on the stove....ladle the hot stuff into a dish, top with crumbs and put it on the serving table...repeat when empty.