Heat the oven to 400 degrees F. Spray an 8-inch x 8-inch pan (or any other 2-quart baking pan) with cooking spray.
Prepare the peppers; cut in half length-wise, remove the stem, seeds and white membrane. Nestle the peppers into your prepared baking dish; set aside.
Heat a large non-stick skillet over medium-high heat. Remove the sausage from its casings, and add to your pan. Use the back of a wooden spoon to break the meat apart into chunks and cook, stirring frequently, until browned.
Place the sausage onto a paper towel-lined plate, and drain off all but 1 teaspoon of the fat. Add the onions and oregano to the pan, and cook until softened and starting to brown, about 5 minutes.
Add the sausage back to the pan along with canned tomatoes, and ½ cup of the mozzarella cheese.
Spoon the mixture into your prepared bell peppers. Cover with foil and bake for 30 minutes. Remove the foil, and top the bell peppers with the remaining ½ cup of mozzarella cheese. Bake for an extra 10 minutes.
Remove from oven and let sit for 5 minutes before serving. Enjoy!