If you want to make this on the stove top, I recommend you used canned beans instead of cooking them from scratch. Process the tomatillo, onion, jalapeño and poblano in the blender until chopped but not completely pureed and simmer this mixture for 8-10 minutes. Add the canned beans and let simmer for 5 more minutes. If the sauce is too thick add vegetable stock or water. Season to taste.