Preheat oven to 375°F (190°C). Lightly grease a baking dish and place the prepared bell peppers inside, cut side up. Tip: Choose peppers with flat bottoms so they stand upright.

Heat olive oil in a skillet over medium heat. Add onion and garlic, sautéing until soft and fragrant, about 3-4 minutes. Tip: Stir frequently to prevent burning.

Add cooked lentils, walnuts, oregano, smoked paprika, salt, and black pepper to the skillet. Stir in tomato sauce and cook for another 2-3 minutes until well combined. Tip: For extra flavor, toast the walnuts lightly before adding.

Spoon the lentil-walnut mixture into the bell peppers, packing gently. Cover the baking dish with foil. Tip: Leave a little space at the top to prevent spillover.

Bake for 25-30 minutes until peppers are tender. Remove foil and bake for another 5 minutes if desired for a crispier top. Tip: Test pepper doneness with a fork—it should pierce easily.

Garnish with fresh parsley before serving. Tip: A sprinkle of nutritional yeast adds a cheesy flavor if desired.