step 1: Show Images
Place a large skillet, preferably cast iron, over medium heat. Add the chorizo to the pan, breaking it up as it cooks. Continue to cook until it is browned and cooked through, about 8 minutes. Add 1 to 2 tablespoons of the olive oil (less if the chorizo was has rendered a lot of fat, more if it was on the lean side). Add the beans to the pan and bring to a simmer.* As the beans cook, mash with a potato masher, the back of a spoon or two forks. Cook, stirring frequently, until the mixture has the consistency of soft mashed potatoes (about 10 minutes after adding the beans). Taste and adjust seasoning with salt as needed.
step 2: Show Images
Brush the split buns lightly with olive oil and toast in a skillet or in the oven. (You may want to hollow out part of the top half of the rolls to make room for all the fillings.) Layer the rolls with sliced queso fresco, then spread with a portion of the bean-chorizo mixture (about ½ cup). Top with the tomatillo salsa and sliced avocado and replace the top halves of the rolls. Serve immediately.