Step 1: Preheat your oven to 400°F (200°C). Line a large baking sheet with parchment paper.
Step 2: On a lightly floured surface, unroll or stretch the pizza dough into a large rectangle, approximately 10x14 inches. If the dough is resistant, let it rest for 5-10 minutes before continuing.
Step 3: Spread the cooked Italian sausage evenly over the dough, leaving a 1-inch border around the edges. Layer with pepperoni slices, shredded mozzarella cheese, thinly sliced red bell pepper, and red onion. Sprinkle generously with Italian seasoning.
Step 4: Carefully roll the dough tightly from one long side to the other, creating a log. Pinch the seams firmly to seal the edges and tuck in the ends to prevent filling from escaping. Transfer the Stromboli to the prepared baking sheet, seam-side down.
Step 5: Using a sharp knife, make 3-4 diagonal slits across the top of the Stromboli. This allows steam to escape during baking. Brush the entire surface with the beaten egg wash for a golden, crispy crust.
Step 6: Bake for 20-25 minutes, or until the Stromboli is golden brown and the cheese is bubbly. If the top browns too quickly, you can loosely tent it with foil.
Step 7: Remove from the oven and let it cool for 5-10 minutes before slicing and serving. This resting period helps the cheese set. Serve with warm marinara sauce for dipping, if desired.