Drain and rinse the beans in a colander. Heat the olive oil in a large skillet and add the onion. Sauté until translucent. Add the garlic and continue to sauté until fragrant. Add the cumin and chili powder and sauté until toasted and aromatic.
Add the pinto beans and the stock and bring to a simmer. Cook the beans for a few minutes to soften and then mash with a potato masher until the beans are mostly smooth with some chunks of beans. Add the lime and season, to taste, with salt and pepper. For a smoother puree, blend the beans in a food processor or blender until desired consistency is reached.
To serve, heat the tortillas in a dry skillet (for a crispier taco, grease the skillet or for burnt edges, toast over an open flame) and fill with refried beans, sliced avocado, tomatoes, lettuce, cheddar, sour cream and salsa.