1. Preheat oven to 375°F (190°C). Cut the tops off the bell peppers and remove seeds and membranes. Lightly brush the peppers with olive oil and place in a baking dish. Bake for 10 minutes to soften slightly.
2. In a skillet, sauté onion and garlic in olive oil until softened. Add ground beef and cook until browned. Stir in tomato paste, beef broth, frozen vegetables, salt, and pepper. Simmer for 5 minutes.
3. Fill each pepper with the beef mixture, pressing it down gently. Top with mashed potatoes and sprinkle with cheddar cheese.
4. Bake for 20-25 minutes, or until the cheese is melted and golden.
5. Serve warm, garnished with fresh parsley.