Heat olive oil in a large pot or Dutch oven over medium heat. Add onion and cook until softened, about 5 minutes.
Add garlic and cook for 1 minute more, until fragrant.
Pour in vegetable broth, add black beans, cumin, chili powder, cayenne pepper (if using), salt, and pepper. Bring to a boil, then reduce heat and simmer for 20 minutes, stirring occasionally.
Remove from heat and stir in lime juice. Taste and adjust seasonings as needed.
Serve hot, garnished with cilantro.
Notes
Serve with a dollop of sour cream or a swirl of plain yogurt for extra creaminess.