Place a rack in top-most position of oven; preheat to 475°F. Place dough on a work surface; drizzle with 1 tablespoon olive oil, turning to coat. Stretch out to a 10" round and cover loosely with plastic wrap, while preparing the toppings.
Carefully lay dough inside a well seasoned cast-iron skillet, using your fingers to help extend dough all the way to the edges. Season with salt, crushed red pepper flakes and crushed garlic and drizzle remaining olive oil on top.
Place the cast iron pan in the preheated oven until crust is lightly golden brown, about 5 minutes.
Remove skillet from the oven, spread marinara over entire dough surface. Top with mozzarella cheese, mushrooms and red onions. Return skillet to the oven and bake on the top rack until crust is deep golden brown around the edges and cheese is bubbly, about 10 minutes. Let cool 5 minutes, then top with sundried tomatoes and basil.