Heat oil in a Dutch oven or large sauté pan over medium heat.
Add onion and red bell pepper. Cook 5–7 minutes until tender, stirring frequently.
Add chili powder, smoked paprika, cumin, salt, and garlic. Stir for 2 minutes until fragrant.
Add lentils, tomato sauce, and vegetable stock. Bring to a simmer, then reduce heat to low-medium, cover, and cook 15–25 minutes, stirring occasionally and scraping the bottom.
When lentils are tender, stir in black beans and kidney beans. Simmer 2–3 minutes until heated through.
Ladle chili into bowls and serve with desired toppings.
