Preheat your oven to 400°F (200°C) for an even bake.
In a large skillet, heat olive oil over medium heat. Sauté red onion for 2-3 minutes until softened, then add mushrooms and cook for 5 minutes. Stir in garlic and spinach until wilted; season with salt and remove from heat.
On a floured surface, roll out pizza dough into a 12x16 inch rectangle.
Spread sautéed vegetables over the dough, leaving space around edges, and top with mozzarella cheese.
Roll the dough from one edge, tucking sides in to seal the filling. Pinch seams to prevent leakage.
Transfer to a parchment-lined baking sheet, brush with egg white, and sprinkle sesame seeds if desired.
Bake for 25-30 minutes until golden brown and cooked through.
Let cool for a few minutes before slicing. Serve warm with marinara sauce.