In a medium skillet, heat 2 tablespoons of the olive oil over medium-high heat. Add the onion and two thirds of the garlic and cook until softened, about 3 minutes. Add the ground beef and cook, stirring to break up the beef, for about 5 minutes, or until browned; season to taste with salt and pepper.
In a medium bowl, combine the mashed potatoes with 1/2 cup Parmesan and the parsley.
In a small bowl, combine the bread crumbs with the remaining 1 tablespoon of olive oil, the remaining one third of the garlic and the remaining 1/2 cup Parmesan.
Stand the bell peppers upright in a baking dish. Stuff each pepper with one quarter of the potato mixture, then one quarter of the meat mixture. Top the stuffed peppers with the garlicky bread crumbs and the bell pepper caps, and drizzle with olive oil. Roast the stuffed peppers until tender, about 30 minutes.