To make the refried beans:In a large skillet heat the olive oil over medium heat. Add the chopped onions and cook until soft. Add the drained beans and mash them using a masher or a fork. Add some of the can liquid to make the beans easy to spread and not too dry. To make assorted-topping Molletes:
Preheat broiler to "Low".
Place halved rolls on a baking sheet and broil until lightly golden, 1 to 3 minutes, depending on your broiler and always keeping an eye on the bread pieces so that they don't burn. Spread each halved roll with 1/4 cup of beans.
For the Chorizo Molletes, spread 1/4 cup of cooked Mexican Chorizo over the refried bean layer of 2 halved bolillo rolls.
For the Ham Molletes, spread 1/4 cup of smoked ham over the refried bean layer of 2 halved bolillo rolls.
Top all the Molletes with shredded cheese. Return to the broiler and broil until the cheese is melted and starting to bubble, 1 to 3 minutes depending on your broiler and always keeping an eye on the bread pieces so that the molletes don't burn. Serve hot and enjoy with your favorite salsa.
To make the Pico de Gallo Salsa:Chop 2 Roma tomatoes, 1/2 cup white or red onions, 1 seeded, serrano chile (or half of one seeded, jalapeno) and a small bunch of rinsed cilantro. I add 2 tsp of olive oil to the salsa also, as it enhances all the flavors. Season to taste with salt and pepper and toss all the ingredients.