Soak the pinto beans in water overnight, or for at least 8 hours. Drain and rinse the beans before use.
In a large pot, add the soaked beans and enough water to cover the beans by 2 inches. Bring the water to a boil and reduce the heat to low. Let the beans simmer for about 1 1/2 to 2 hours, or until the beans are tender. Drain the beans and reserve the liquid.
In a large skillet, heat the lard or oil over medium heat. Add the onion and garlic and cook until softened, about 5 minutes.
Add the cooked beans to the skillet and mash them with a potato masher or a fork until they are well mashed. If the beans are too thick, you can add some of the reserved liquid to reach the desired consistency.
Season the beans with salt and pepper to taste. Cook for an additional 5 minutes, or until the beans are heated through.