In a large stock pot over medium-high heat, cook the chopped onion, carrots, celery, garlic and red pepper in a pat of butter or coconut oil for about 10 minutes, until tender.
Transfer the cooked vegetables to a high-speed blender, and add in the water and 1/2 a can of black beans. Process until a smooth puree is formed.
Return the puree to the stock pot over medium-high heat, and add the remaining black beans, cumin, salt and lemon juice. If you'd like a thinner soup, feel free to add a bit more water, and any additional seasoning, to taste.
Serve piping hot, with a dollop of fresh salsa on top!