Coat a large bowl with nonstick cooking spray, remove dough from packaging, and place inside. Cover the top of the bowl with plastic wrap and rest on the counter while you prepare the caramelized onions and fennel.
Preheat the oven to 420°, and place the rack in the lowest setting of the oven. (Don't skip this preheating step. It's important that your oven is nice and warm before putting your pizza in there. If you have a pizza stone, use it.)
Heat the oil in a nonstick skillet over medium heat. Add the onion and fennel. Cook, stirring frequently for 5 minutes. Turn down the heat to low and cover with a lid or foil. Cook until onion and fennel are tender, stirring occasionally, about 35 more minutes. Remove from heat, and season with salt.
To make the pizza: Dust your counter or other clean work surface lightly with flour (any flour will do). Remove dough from the bowl and place on your work surface. Using a rolling pin or your hands, shape dough into a 12" circle.
Transfer dough to a piece of parchment paper. Spread the pizza sauce over the crust and top with the spinach, caramelized onions and fennel, tomatoes, and the cheese. Sprinkle with Italian seasoning.
Keep the pizza on the parchment, and either transfer it to the oven directly onto a pizza stone or set it on an inverted cookie sheet before placing in the oven. Bake for about 15 minutes, until cheese is melted and crust turns golden brown.
Remove from the oven, cut, and enjoy.