Heat the olive oil in a large pot over medium heat. Add the onion, garlic, and red bell pepper. Cook for 4 to 5 minutes until the onion turns soft.
Stir in the cumin and chili powder. Cook for 30 seconds until the spices smell fragrant.
Add the rinsed lentils, diced tomatoes, and vegetable broth. Bring the mixture to a boil, then lower the heat to a simmer.
Cover the pot and cook for 25 minutes, stirring once or twice, until the lentils are tender.
Stir in the black beans and cook uncovered for 5 more minutes to heat them through. Season with salt and pepper to taste.
Ladle the stew into bowls and top with fresh cilantro and a squeeze of lime.