Place the ground sausage in a skillet over medium heat. Cook, stirring to break it up, until browned and cooked through, about 3–4 minutes.
Remove the sausage from the pan but leave the drippings. Add the chopped onion and green pepper.
Cook, stirring occasionally, until the onion and pepper are soft, about 4–5 minutes.
Lay the Italian bread on a flat surface. Spread the butter evenly over the top of each slice and sprinkle with Parmesan.
Heat a griddle over medium heat (you can also do this in two large skillets).
Add 4 slices of the bread, buttered-side down. Divide the pizza sauce over the top, then divide ¾ of the cheese over the top of the marinara.
Divide the pepperoni, sausage, onion, and pepper over the top of each slice. Sprinkle the remaining cheese over the top.
Top with the remaining bread slices, buttered-side up. When the bottom of the bread is lightly golden brown, carefully flip each sandwich. Continue cooking until the bread is golden brown on the other side and the cheese has melted.