Preheat oven to 350°
In a frying pan over medium heat, cook the burger until evenly browned and drain.
While the burger is cooking, cut the tops off the peppers (save any that remain whole but not necessary to do so) and remove membranes and seeds from the insides. Place peppers in a deep baking dish with the holes facing up. If peppers do not site evenly, cut a thin piece off the bottom to make level. Do NOT cut a hole in the bottom of the pepper.
Reserve half of the tomato sauce and italian seasoning. Mix the rest of the ingredients in a bowl.
Spoon evenly into peppers.
Place any tops of peppers that remained in tact back on top of the stuffed pepper.
Mix remaining tomato sauce and italian seasoning in a bowl.
Pour mixture evenly over peppers. (If making peppers ahead, save this final step until you are ready to cook).
Place a few pieces of onion on top of each pepper.
Cover and bake 1 hour. Baste every 15 minutes.