Cook Lentils: Rinse the lentils and place them in a pot with vegetable broth or water. Bring to a boil, then reduce heat and simmer for about 20-25 minutes, or until lentils are tender but not mushy. Drain any excess liquid.
Prepare Filling: In a large skillet, heat olive oil over medium heat. Add diced onion and cook until translucent, about 3-4 minutes. Add minced garlic and diced bell pepper, and cook for another 2-3 minutes until bell pepper is tender.
Add Seasonings: Stir in chili powder, ground cumin, paprika, salt, and pepper. Cook for another minute to toast the spices and blend flavors.
Combine Lentils: Add cooked lentils to the skillet with the onion and bell pepper mixture. Stir well to combine and let the flavors meld together for a couple of minutes.
Assemble Tacos: Warm tortillas in a skillet or microwave. Spoon the lentil filling onto each tortilla. Top with avocado slices or guacamole, salsa, shredded lettuce or cabbage, chopped tomatoes, and any other desired toppings.
Serve: Serve the tacos with lime wedges on the side for squeezing over the top.