Preheat your oven to 400°F (200°C) and line a baking sheet with parchment paper.
In a large mixing bowl, combine the spinach, bell pepper, zucchini, red onion, mozzarella cheese, ricotta cheese, garlic powder, oregano, and crushed red pepper flakes. Season with salt and pepper to taste. Mix well until all ingredients are combined.
Roll out the pizza dough on a lightly floured surface into a rectangle about 14x10 inches.
Spread the veggie and cheese mixture evenly over the rolled-out dough, leaving a small border around the edges.
Starting from one long side, carefully roll the dough inward to form a log, sealing the edges as you go. Pinch the seams to secure the filling inside.
Place the stromboli seam-side down on the prepared baking sheet. Brush the top with beaten egg and drizzle with olive oil.
Make a few slits on the top of the stromboli for steam to escape.
Bake in the preheated oven for 20-25 minutes or until the dough is golden brown and fully cooked.
Remove from the oven and let it sit for 5 minutes before slicing.
Notes
Serve warm, sliced into pieces, and garnished with fresh basil or parsley. Pair with marinara or garlic dipping sauce on the side for an extra flavor boost!