Preheat your oven to 400°F (200°C).
Line a baking sheet with parchment paper.
On a lightly floured surface, roll out the pizza dough into a rectangle, approximately 12x16 inches.
Spread the marinara sauce evenly over the rolled-out dough, leaving a small border around the edges.
Layer the shredded mozzarella cheese, pepperoni slices, diced bell pepper, and diced onion over the sauce.
Sprinkle Italian seasoning, salt, and pepper over the filling.
Starting from one long edge, carefully roll up the dough tightly, like a jelly roll, sealing the edges as you go.
Place the rolled Stromboli seam-side down on the prepared baking sheet.
Beat the egg in a small bowl and brush the egg wash over the top of the Stromboli.
Make a few slits on top of the Stromboli with a knife to allow steam to escape.
Bake in the preheated oven for about 25 minutes, or until golden brown.
Let it cool for a few minutes before slicing and serving.