In a large skillet over medium heat, add the extra virgin olive oil. Once hot, add the chopped red onion and sauté until translucent, about 3-4 minutes. Next, add the sliced cremini mushrooms and crushed garlic, cooking until the mushrooms are tender and lightly browned, around 5-7 minutes. Stir in the baby spinach and cook until wilted, about 2 minutes. Season the filling with kosher salt, then remove from heat and let it cool slightly.
Preheat your oven to 400°F (200°C). If using refrigerated pizza dough, roll it out on a floured surface into a rectangle, approximately 12x16 inches. If you’re making homemade dough, follow your favorite recipe to the point of rolling out.
Once the filling has cooled, spread it evenly over the rolled-out pizza dough, leaving about an inch of space around the edges. Sprinkle the shredded mozzarella cheese over the filling, ensuring even distribution.
Starting from one long edge, carefully roll the dough over the filling, tucking it in as you go to create a tight log shape. Pinch the seams to seal and ensure that the filling doesn’t escape during baking.
Transfer the stromboli to a baking sheet lined with parchment paper. Brush the top with beaten egg white for a glossy finish and sprinkle sesame seeds, if using. Using a sharp knife, make 3-4 slits on the top of the dough to allow steam to escape while baking.
Bake the stromboli in the preheated oven for 25-30 minutes, or until golden brown. The crust should be firm to the touch and the filling will be bubbly.
Let the stromboli cool for a few minutes before slicing it into thick pieces. Serve with a side of marinara sauce for dipping. Enjoy the delicious flavors of this Veggie Stromboli!
