Preheat the oven to 375°F.
Roll out the pizza dough on a floured surface to form a rectangle.
In a mixing bowl, combine the chopped spinach, ricotta cheese, mozzarella cheese, parmesan cheese, minced garlic, red pepper flakes, salt, and black pepper.
Spread the spinach mixture over the pizza dough, leaving a 1-inch border around the edges.
Roll up the pizza dough, starting from one of the long sides, and pinch the edges to seal.
Brush the egg over the top of the stromboli.
Bake the stromboli for 30-35 minutes, or until golden brown.
Before slicing and serving, for 5 minutes let the stromboli cool.