In a large skillet over medium to high heat, add oil and sauté bell pepper, onions, and garlic until fragrant and onion is translucent.
Add lentils and tomato paste. Stir well to evenly coat and cook the tomato paste.
Add ketchup, stock, sugar, mustard, chili, Worcestershire, and pepper. Stir well to combine and cook for 10-15 minutes to heat through and allow sauce to thicken.
Serve hot over toasted burger buns.
