Preheat the oven to 425°F (220°F). Line a large baking tray with foil and spray with cooking spray. Spray a 13x9 inch glass or ceramic baking dish with cooking spray and set aside.
Put the flour, lightly beaten eggs, and Panko into three separate shallow dishes such as pie plates or cake pans. Season all three with salt and pepper. (Approximately ¼ tsp each in each dish.)
Dip each cauliflower floret in flour, shaking off the excess. Dip into the egg and then coat with the breadcrumbs. It is best to try to keep a "wet" hand and a "dry" hand to do this, or you can use tongs. Once the floret is covered with breadcrumbs, place it on the baking tray. Repeat with all the florets. You should have approximately 36-40. Spray the florets generously with cooking spray, preferably olive oil or avocado oil spray.
Place the cauliflower in the oven and bake for approximately 20-25 minutes, until the breadcrumbs are crispy and the florets are beginning to get brown.
While the cauliflower is cooking, make the tomato sauce if you haven't already. Prepare the cheeses and the basil.
Remove the cauliflower from the oven. Put ¾-1 cup of the sauce in the bottom of the prepared baking dish. Arrange half of the florets evenly over sauce. Dollop/spread half of the remaining sauce on and around the cauliflower and then top evenly with half of the Pecorino/Parmesan and half of the mozzarella cubes and half of the sliced basil. Nestle the remaining florets in between the other ones. Top with the rest of the sauce, the remaining Pecorino/Parmesan cheese and sliced basil.
Reduce oven temperature to 400°F (200°C). Place casserole in the oven and bake for 20-25 minutes or until the casserole is bubbling and the cheese is golden brown. Let sit for a few minutes and then top with the basil leaves and serve immediately.