Preheat oven to 375°F (190°C). Lightly grease a baking dish large enough to hold 4 peppers upright.

Cut the tops off the bell peppers and remove seeds and membranes. If needed, trim the bottoms slightly so they stand upright, being careful not to create holes. Place peppers in the prepared baking dish.

Heat olive oil in a large skillet over medium-high heat. Add diced onion and cook for 3-4 minutes until softened. Add garlic and cook 1 minute more. Add ground lamb, breaking it up with a spoon, and cook until browned, about 6-8 minutes. Drain excess fat if needed.

Add cooked rice, diced tomatoes, oregano, salt, and pepper to the lamb mixture. Stir in half the feta cheese, parsley, and mint. Cook for 2-3 minutes until heated through and well combined. Taste and adjust seasoning.

Spoon the lamb and rice mixture into each pepper, filling them generously and mounding slightly on top. Pour 1/4 cup of water into the bottom of the baking dish.

Cover the baking dish with aluminum foil and bake for 35 minutes. Remove foil, sprinkle remaining feta cheese on top of each pepper, and bake uncovered for an additional 10 minutes until peppers are tender and cheese is slightly golden.

Remove from oven and let rest for 5 minutes. Garnish with additional fresh parsley and mint before serving.