Place the bolillo roll halves cut side up on a baking sheet and spread each with 1/2 tablespoon of butter or margarine. Bake in preheated oven for 5 minutes.
Place refried beans in a microwave-safe bowl and microwave on high for 1 to 2 minutes until warm.
Divide the refried beans among the toasted rolls and spread evenly to the edges. Sprinkle each roll with 2 tablespoons of shredded mozzarella. Return rolls to oven and bake until cheese is melted, 7 or 8 minutes.
Remove from oven and top each roll with the sliced tomatoes, avocados, red onion and chopped cilantro. Sprinkle each roll with a pinch of cumin and a pinch of salt. Drizzle each roll with 1/4 teaspoon of olive oil.