In a large pan, heat oil over medium heat. Add onions and sauté for 2-3 minutes.
Add bell peppers and continue to cook for about 3-4 minutes more. Add garlic and sauté unlit fragrant, about 30 more seconds.
Stir in black beans, lentils, crushed tomatoes, water, brown sugar, and yellow mustard. Bring to a boil, reduce the heat, cover, and simmer for 30-35 minutes or until lentils are cooked through (this may take a bit longer due to cooking in tomato, so continue to cook until soft and cooked through)
Uncover and continue to cook until reduced to desired consistency, about 10-15 minutes more.
Serve on buns, topped with cheese, onions, or any toppings you like! Enjoy!