Add the olive oil to a large skillet set over medium heat. Add the onion and garlic and saute until tender and translucent. Add beans, cumin, salt, and pepper to the skillet . Add reserved liquid a little at a time until beans are at desired consistency and not dry. Cook until beans are heated throughout. Mash beans with a potato masher or the back of a wooden spoon to your desired texture. Remove from heat and serve.