If you can find them, buy 2-3 cans of Diamond brand small pearl onions. They seem to getting scarce, so good luck. Frozen are probably easier to find, and they’ll work well, but may need to cook a little longer. If you have an extra hour, buy fresh pearl onions, make a small cut in end, drop into boiling water, and peel. These will take even longer to cook, so plan on that extra time in the oven.
(I haven’t been able to find the canned onions lately, so I made creamed leeks and fennel with fennel pollen this year.)
Make a simple white sauce. This means melting some butter over medium heat, adding the same amount of flour and cooking that for a few minutes. Slowly add milk. If you used a quarter cup each of butter and flour, use about a cup of milk. If this is too casual, consult any cookbook for a more precise recipe.
As the sauce comes to a boil and thickens, add some grated or crumbled cheese. It can be almost anything, but in recent years I’ve found that an extra sharp cheddar mixed with some kind of blue-veined cheese tastes best. A shot of bourbon or brandy is also good here (in the sauce, that is, but feel free).
Melt some butter in a skillet, and add about a cup of your home made bread crumbs. Cook gently until they brown a bit.
Open the cans, drain the water off of the onions, and pour them into a shallow baking dish. Pour the sauce over, and top with the bread crumbs. Bake the onions at 350 until browned and bubbly.