Heat the olive oil in a large pot over medium heat. Add the onion and cook for 3 minutes until it starts to soften.
Stir in the garlic, cumin, and chili powder. Cook for 1 minute until the spices smell fragrant.
Add the lentils, potatoes, diced tomatoes, vegetable broth, salt, and black pepper. Stir everything together.
Bring the mixture to a boil, then reduce the heat to low. Cover the pot and simmer for 30 minutes, stirring once or twice, until the lentils and potatoes are tender.
Remove the lid and let the stew simmer uncovered for 5 more minutes to thicken slightly.
Stir in the chopped cilantro right before serving. Ladle into bowls and add a squeeze of lime.