Heat a medium skillet over medium-high heat. Add 2 tablespoons oil and the sausage and cook, breaking apart the sausage with a wooden spoon, about 5 minutes. Add the salt, red pepper and onion and cook, stirring often, until the peppers and onions are soft and the sausage begins to brown, about another 5 minutes. Set aside to cool slightly.
Divide 1/4 cup marinara sauce between each half of one of the ciabatta rolls and top with a quarter of the mozzarella slices over the sauce. Place 2 slices of the pepperoni on the top half and fill the bottom half with a quarter of the sausage mixture. Sprinkle the sausage with 1/4 cup of the Parmesan. Sandwich together and continue with the remaining ingredients.
Heat a large skillet over medium heat. Add 2 tablespoons oil and 2 of the sandwiches top-side down to the skillet. Place a small pan on top and press down gently to flatten slightly. Cook until golden brown and crispy, 3 to 4 minutes. Flip the sandwiches and return the pan to the top, again pressing gently. Cook until the bread is golden and the cheese is melted, about 3 minutes. Add 2 tablespoons oil to the skillet and continue with the remaining sandwiches. Allow the sandwiches to cool to room temperature and then chill them in the refrigerator until ready to serve.
Before serving, slice the sandwiches into wedges like you would a pizza.