Heat oven rack over high heat; place the rack in the center of the oven. Spread the butter over the cut of each bread. Place the loaves on an aluminum lined baking sheet; cook on the grill for approximately 1 minute, or until the bread is browned and toasted.
Spread the beans evenly on each loaf using a spoon. Sprinkle with cheese without forgetting to cover the edges. Grill 3 to 5 minutes more or until cheese is melted and beans are heated through.
Garnish on top with avocado and Pico de Gallo Sauce. Serve right away. Note: Bolillos are traditional Mexican muffins, but you can use crispy French-style muffins or Italian-style soft bread.