Cook the sausage in a cast iron skillet over medium heat until it starts to brown slightly, then add onions and cook until translucent.
Cook sausage mixture for a few more minutes, then add serrano pepper and black beans. Keep warm.
Toast rolls and scramble eggs lightly in a small skillet over medium-low heat. Season eggs with salt and pepper.
Assemble tortas by adding cooked sausage mixture to a toasted roll. Top with a handful of cheese and scrambled eggs. Repeat with remaining rolls.
For on the go: Wrap torta in a sandwich wrapper, then slice in half and serve while warm!