In a large bowl, combine the flour, instant yeast, and salt. Gradually add the warm water and olive oil, mixing until a dough forms. Knead the dough on a floured surface for about 5 minutes until smooth.
Place the dough in a greased bowl, cover with a kitchen towel, and let it rise in a warm place for about 1 hour, or until doubled in size.
Preheat your oven to 400°F. Line a baking sheet with parchment paper.
Once the dough has risen, punch it down and roll it out on a floured surface into a rectangle, about 1/4 inch thick.
Spread the marinara sauce evenly over the dough, leaving a 1-inch border around the edges.
Layer the mozzarella cheese, pepperoni, bell peppers, and Parmesan cheese on top of the sauce. Sprinkle with Italian seasoning.
Starting from one long side, carefully roll the dough into a tight log. Pinch the edges to seal and tuck the ends under.
Place the rolled stromboli seam-side down on the prepared baking sheet. Brush the top with the beaten egg for a golden finish.
Bake for 20-25 minutes, or until the stromboli is golden brown and cooked through. Let it cool for 5 minutes before slicing.
Serve warm with extra marinara sauce for dipping.