Prep the dough - several hours in advance of making the pizza, remove the dough from refrigerator to bring to room temperature. This will allow the dough to be more pliable and not shrink back when cooking.
Preheat oven to 500 (or higher if you can), ideally for at least 30 minutes prior to baking your pizza.
Carmelize the Onions - in a large saute pan or dutch oven, combine about 2 Tbsp olive oil and the onions, seasoning with kosher salt and black pepper. Cook over medium heat for about 25-30 minutes, until they have shrunk and developed a golden caramel color. Set aside.
Cook the Sausage & Spinach - in a cast iron skillet, brown the sausage, breaking it up into small pieces as you go. Set aside to cool, and use the same pan to wilt down the baby spinach, tossing with salt and pepper, allowing the heat of the lower layer of spinach to wilt the other leaves. Once cooled, wring it out, removing all excess moisture - set the tiny ball of cooked spinach to the side - this will not result in a lot, but the flavor will be very concentrated .
Prep a 1/2 baking sheet (12'x18") with a little olive oil and ground cornmeal to prevent sticking. Alternatively, you can use a preheated baking stone without the oil and cornmeal. Roll or stretch out your dough to fit the surface of your baking pan, and lay it out.
Layer your toppings, starting with the Marinara or pizza sauce (to your liking), then the sausage and carmelized onions, and spinach, then finally, the mozzarella cheese. Sprinkle the Parmesan on top of it at the end, along with about a tsp of red pepper flakes (omit if you don't want the heat).
Bake in middle rack of oven for at least 8 minutes, up to 10, depending on the level of doneness you are seeking. The crust should be crisp on the edges, and fully cooked in the middle of the pizza.
Set aside to cool and eat while warm, or anytime you like - this makes a great breakfast on the go too!