Preheat a skillet with butter to medium low heat.
Char the onion slices in a broiler for ~5m. (And maybe do the plantain with it? Experiment here.)
Assemble the sandwich: spread the inside of the split roll with mustard and drizzle with hot sauce to taste. Fill with sliced queso blanco, plantains, red onion, avocado, tomato and more queso blanco.
Add the sandwich to the hot skillet and press with a heavy pan. Cook until golden, 3 to 4 minutes per side.