8 ounces butter
4 ounces flour
2 cups milk, warmed
1 sprig fresh thyme
6 medium yellow onions, whole
4 medium yellow onions, diced
1 cup white wine
1/2 cup breadcrumbs, toasted
4 bacon strips, browned in the oven and finely chopped
Salt and pepper to taste
Preheat the oven to 350 degrees. Make a béchamel sauce: Melt four ounces of butter in a large saucepan set over low heat. Slowly whisk in the flour and cook for five minutes. Gradually whisk in the warm milk. Add the sprig of thyme. Simmer until thickened, whisking constantly, for about five minutes. Strain and set aside.
Boil six of the onions until you can pierce them easily with a knife. Peel the onions and using a paring knife cut off the top and the trim root end (leaving it intact). Carve out the inside of the onion, leaving about three layers of outer skin. Set the skins in a baking dish.
Melt four ounces of butter in a saucepan over low heat. Sweat the diced onions until translucent, then add the white wine. Let the mixture simmer until it is almost dry, taking care not to let the onions brown. Remove from the heat, and add 1/3 of the béchamel sauce to the onions. Season with salt and pepper.
Fill the onion shells with the onion stuffing. Pour the remaining sauce over and around the onions in the baking pan. Sprinkle the tops with breadcrumbs and bacon crumbles. Bake the onions in the oven until they are heated through and the tops are bubbly and lightly browned, about 20 minutes.