In a large skillet, heat the olive oil over medium heat. Add the diced onion and bell pepper, cooking until they are softened, which should take about 5 to 6 minutes.
Add the minced garlic to the skillet and cook for an additional minute.
Stir in the tomato paste, smoked paprika, chili powder, and a pinch of salt. Allow this mixture to cook for 2 minutes to enhance the flavors.
Incorporate the cooked lentils, tomato sauce, ketchup, mustard, and soy sauce into the skillet. Mix everything together thoroughly.
Let the mixture simmer for 5 to 7 minutes, or until it thickens and is heated through. Taste and adjust the seasoning as needed.
Spoon the lentil mixture onto toasted buns and add any desired toppings, such as vegan cheese slices, which melt nicely.
