Preheat oven to 350º and line a cookie sheet with parchment (this is not entirely necessary, but I don't like washing cookie sheets).
Spoon a layer of beans onto half of the shells, cover with onions and cheese. Spoon a layer of beans onto the other half and make a bean sandwich and spoon beans onto the top of those shells. Cover with onions and more cheese.
Bake for 10 minutes and then check cheese for the desired doneness. If you like it meltier, leave 'em in for three to five more minutes.
Remove from oven and plate; top with tomatoes, lettuce, salsa, sour cream, and any other thing you'd like!
