In a skillet, sauté onion, green pepper, and garlic at medium heat until fragrant, about 5 minutes. Add cooked lentils, tomato sauce, chopped tomato, cumin, oregano, salt, and pepper. Stir to combine.
Bring to a simmer and cook until the sauce thickens, about 8-10 minutes, stirring occasionally. Remove from heat and stir in olives and raisins, if using. Remove from heat.
Add picadillo evenly into each bell pepper then transfer into a baking dish. Add 1 cup of water to the baking dish then cover with aluminum foil. Bake stuffed bell peppers for 15-20 until the peppers are tender. Serve immediately. Enjoy!