Preheat the oven to 350 degrees F (175 degrees C). Spray the bottom and sides of a 2 1/2-inch deep, 9 1/2-inch round springform pan with nonstick spray, then evenly dust with cornmeal.
Roll out and stretch pizza dough to a diameter of about 15 1/2 inches. Carefully place it in the prepared pan to cover the bottom and sides with some overhang, re-stretching as needed.
Toss mozzarella, Cheddar, and Parmesan cheeses together to mix. Set aside 1 cup in the refrigerator for later. Sprinkle 1/2 of the remaining cheese over the dough. Layer with sun-dried tomatoes, sausage, mushrooms, sweet pepper, and olives, in that order.
Prepare custard: Whisk eggs in a 4-cup liquid measure. Add cream, eggs, basil, oregano, fennel seeds, garlic salt, crushed red pepper, and garlic powder and whisk until well combined.
Pour 1/2 of the custard mixture over the layered ingredients. Top with remaining (non-refrigerated) cheese, then pour the remaining custard evenly over top. Roll the dough overhang inward over the cheese and custard.
Place the springform pan on a large sheet of foil, and gather the foil up and around the pan in case it leaks. Transfer to the preheated oven and bake for 1 hour 15 minutes. Tent the top with foil to prevent excessive browning, and bake until fully set, about 45 minutes longer.
Carefully remove from the oven. Sprinkle with reserved 1 cup cheese and pepperoni slices. Pop back into the oven and bake until cheese melts, about 10 minutes.
Remove from the oven and place on a cooling rack. Tent loosely with foil and let rest for about 2 hours. (It will stay hot and be completely set after 2 hours.)
Release the ring. Slice through the outer crust with a serrated knife, then a sharp chef's knife to finish slicing the pieces.