Prep. Preheat the oven to 375°F. Spray a 13x9-inch baking dish with cooking spray.
Trim the onions. Cut a ¼-inch slice from the top of each onion. Trim a small amount from the bottom end so each onion stands upright.
Microwave the onions. Place half of the onions in an 8x8-inch microwave-safe dish. Add 1/2 cup water and cover. Microwave on HIGH for 12 minutes, or until the onions are tender. Carefully remove onions from the dish and cool slightly. Repeat with remaining onions and water.
Hollow out the centers. When cool enough to handle, carefully remove the inner layers of the onions with a paring knife, leaving about ¼-inch around the edges. I remove about 3 to 4 layers from each onion, depending on the size of the onion. Chop and reserve 1 cup of the onion for the filling. Set aside the remaining onion for another use.
Fill the pan. Next, spread 1 cup of tomato sauce into the bottom of the prepared baking dish. Arrange prepared onion shells over tomato sauce. Sprinkle with salt and pepper and set aside.
Cook the sausage. Heat a large nonstick skillet over medium-high heat. Add sausage; cook 4 to 5 minutes or until crumbled and no longer pink, stirring occasionally. Drain the excess grease.
Make the filling. Add the reserved 1 cup of chopped onion, then garlic, and cook for 2 to 3 minutes or until fragrant. Stir in undrained tomatoes, remaining tomato sauce, ½ cup cheese, and bread crumbs. Stir to combine.
Stuff the onions. Spoon the sausage mixture into the prepared onion shells.
Bake. Cover the baking dish with aluminum foil. Bake for 15 to 20 minutes or until heated through. Remove from oven and sprinkle the remaining ½ cup cheese on top of the onions. Afterward, serve the stuffed onions with a little sauce from the dish.