Preheat a panini press or a skillet over medium heat.
Lay out the slices of sourdough bread on a clean surface.
Spread 2 tablespoons of marinara sauce on two slices of bread.
Layer the shredded mozzarella cheese evenly over the marinara sauce.
Add the pepperoni, black olives, and bell peppers on top of the cheese.
Sprinkle Italian seasoning, salt, and pepper over the toppings.
Top with the remaining slices of bread to form two sandwiches.
Brush the outside of each sandwich with olive oil.
Place the sandwiches in the panini press or skillet. If using a skillet, press down with a heavy pan or spatula.
Cook for about 4-5 minutes, or until the bread is golden brown and the cheese is melted.
Remove from heat, slice in half, and serve warm.