Preheat oven to 400º F. Heat a 12-inch cast iron skillet over medium-high heat. Add ground beef, onion, and seasonings from fennel seeds to salt. Cook until meat is browned, 7–10 minutes, breaking up meat and stirring during cooking. Drain excess oil and place mixture into a bowl. Allow skillet to cool slightly and wipe off with paper towel.
Evenly sprinkle cornmeal in skillet (this adds flavor, texture and helps prevent sticking). Place dough from one can in bottom and press up into sides to reach the top of the skillet (you may need to cut of square edges or mold into a circle shape). Spread on sauce, top with meat mixture, sprinkle with mozzarella, mushrooms, bell peppers, then top with Parmesan cheese. Place remaining dough sheet on top, then press into edges of bottom dough to seal in all ingredients.
Cut a few small slits on top to let out steam while baking. Bake 25 minutes, until crust is golden brown. Remove and let cool 5–10 minutes before slicing. Top with optional basil leaves.