In a large saute pan over medium heat, pour the oil and heat until hot but not smoking, about 1 to 2 minutes. Add the onions and let them saute for about 3 to 4 minutes, until they have softened, become translucent and have started to slightly brown along the edges.
Add a cupful of beans at a time, over the sauteed onion, and mash with a masher, like a potato masher, until you have added all of the beans.
Let them season and thicken for about 8 to 10 minutes, and continue mashing and stirring, until they have achieved a thick puree consistency and are somewhat pasty looking.