Add the bean puree over the sauteed onion, and let them season and thicken for about 10 to 15 minutes, stirring occasionally. If the beans are bubbling way too much, lower the heat a little to medium-low heat. Let them continue thickening, until they have achieved a thick puree consistency and are somewhat pasty looking. It depends on your liking, some people like them thicker and some people like them with a softer consistency. While it depends on how you like them, they are mostly cooked until they can hold a shape when scooped with a spoon.