Heat a large skillet over medium heat. Add 2 tablespoons of olive oil. Once hot, add 1 diced brown onion, 1 diced bell pepper, and 2 minced garlic cloves. Sauté until the vegetables are soft and the onions are translucent, about 5 minutes.

Stir in 1 tablespoon of chili powder, adjusting the amount to taste, and cook for another minute until fragrant.

Add 2 tablespoons of tomato purée, 1 can of chopped tomatoes, and 1 can of drained and rinsed red kidney beans to the skillet. Simmer the mixture for 20 minutes, stirring occasionally, until thickened.

While the chili simmers, prepare the taco assembly ingredients: chop additional fresh tomatoes and shred some lettuce.

Warm 8 tortillas in the microwave or on a dry skillet just before serving.

To assemble the tacos, spoon the vegetarian chili onto each warmed tortilla. Top with chopped tomatoes, shredded lettuce, and any other desired toppings.

Serve immediately, allowing diners to add more toppings as they prefer.