Preheat oven to 400 degrees 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. Place dough from one can on the bottom and press up into sides to reach the top of the skillet. Spread sauce on top, then add meat mixture, mozzarella, mushrooms, bell peppers, and finish with Parmesan cheese. Place remaining dough sheet on top, and 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 basil leaves.