Add 1 cup lentils and 3 cups water to a pan. Set on medium heat and bring to a boil, then turn the heat to low and cook until the lentils are soft for about 30 minutes. Set aside.
Meanwhile, heat a large skillet over medium heat. Once hot, add oil, onion, garlic, and bell pepper. Season with a pinch of salt and pepper and stir to combine.
Cook stirring occasionally for about 10 minutes, or until the peppers and onions are tender.
Add tomato sauce, sugar, soy sauce, chili powder, cumin, and smoked paprika. Stir to combine.
Add cooked lentils to the mixture and continue cooking over medium heat for about 5-10 minutes.
Lightly toast the burger buns and fill the buns with the lentil mixture and sliced onions, add some fresh basil and serve warm.