In a pot, combine the first six ingredients; simmer for 10 minutes.
Add half a can of beans, salt and cumin; cook for 5 minutes.
Puree soup (use an immersion blender which makes it easy to do it right in the pot).
Add the rest of the beans to the soup.
Combine the cornstarch with 1 ½ tablespoons of water.
Add the lemon and the cornstarch to the soup; cook until thickened.