Hollow out onions (a melon-baller works well -- leave at least 2-3 "layers of rings" so onions can support filling) then blanch the "shells" until they are softened, about 9-10 minutes.
Saute the onion pieces removed (from about 2 onions) in a little butter. Mix this with enough breadcrumbs to "glue or bond" the mixture together and then add enough cream to "wet" or keep the mixture moist.
Add cooked rice, and salt & pepper to taste.
Stuff onion-rice mixture back into onion shells, Garnish with more grated swiss and thyme. Bake under moderate heat just to reheat and melt cheese topping.