Ideally you start with a real mexican bolillo, which is a roll with a thick crust and white soft interior, shaped like a yam. If you can’t find that, a small baguestte or other thick-crusted roll will have to do.
Cut the rolls in half, leaving a top half and bottom half. Turn the top piece crust-down so you now have two bottom halfs. Smear a layer of refried beans on each half-roll, about a quarter inch thick. Lay slices of white cheese, either Oaxaca, Manchego, Monterrey jack, or Mozzarella over the top, covering the entire surface. Put a thin slice of tomato on top of the cheese in the center of the roll. Sprinkle a little Mexican oregano over the whole thing. Bake at 350 until the cheese on top starts to brown a bit. Eat.