Peel the onions and cut off the tops about one-fourth the way down. Scoop out centers of onions, leaving quarter-inch-thick onion shells. Chop onion centers.
In a skillet, cook bacon until crisp. Remove and crumble. Pour off fat, but don't wipe the pan. Cook chopped onion in remaining fat until tender.
Lower the heat to medium-low and stir in all the other ingredients and the bacon. Cook until all the moisture has been absorbed and the mixture is beginning to dry.
Fill onion shells loosely with spinach mixture. Blend the topping ingredients and sprinkle generously on top of the onions.
Arrange the stuffed onions in muffin tin. Cover with foil. Bake at 400 degrees for 20 minutes, then remove foil and bake until the topping browns.
