First off, did you know that you can buy frozen pearl onions? It's greatness! No pealing, no nothing. So, get a package of those. Boil them on the stove as you would any frozen vegetable, drain and keep warm. Then, in another pan, melt a tbsp of butter and add an equal amount of flour. Blend with a whisk and then add in half and half or milk. Keep it cooking until it's thick, add a little pepper and put in the onions.
While the onions are still boiling, throw some bacon in a skillet and cook it. If you don't know how to do this, I'm sad for you. Also, while you were at the grocery store getting the pearl onions, you should have picked up some fun bread. I can't remember the type that's in the picture, but at the moment, I love ciabatta. Also make sure you've got mayo and some kind of green lettuce-y thing. Oh, and I skip the tomatoes.
Put the sandwich together and throw the onions on the side and you're good to go! By the way, you'll have terrible onion breath, but it's worth it.