Sauté onion, celery, pepper, and garlic in a large saucepan over medium heat for 5 minutes. Do not let the veggies brown.
Add water, 2 cans of beans, bullion cubes, salt, and cumin. Bring to a boil then reduce heat and simmer for 10 minutes.
Purée the soup using an immersion blender or by pouring into a blender in batches. Be very careful not to fill the blender too full and use a hand towel to hold the lid down on the top of the blender so that the hot liquid does not spray out.
Add the remaining cans of beans to the soup along with the lemon juice. Combine the cornstarch with 4 tablespoons of water in a small bowl, then add this slurry to the soup and bring it back up to a simmer and cook until thick.