Preheat the oven to 400° F and grease a muffin tin very well, including around the top of each cup since the cheese may spill out a little.
Use your hands to stretch the dough into an 11x16-inch rectangle on a piece of parchment paper. (This helps prevent the dough from sticking to the counter. If it's hard to stretch, let it rest for 5 minutes and try again. Sometimes the gluten needs to rest.)
Spread pizza sauce over the dough to cover, then sprinkle on the spinach, mozzarella, and Parmesan. Starting on one long side, roll the dough up carefully and fairly tightly to enclose the toppings. (It might be a little messy, but it's okay! Use the parchment paper to help you roll.)
Use a serrated knife to cut into 12 even slices. I start in the center, then make a cut in the center of each piece, and then finish slicing to get the most even results.
Place each slice into a prepared muffin cup and bake for 22-26 minutes until the dough is baked through, the cheese is melted, and the tops are golden.
Let cool for 3-5 minutes in the pan to allow the cheese to firm up slightly, then transfer to a wire rack or a plate to serve. (You may need to use a a knife around the edges of the pan if the cheese spilled over.)
Store in an airtight container for up to 3 days in the fridge or in a zip-lock freezer bag with as much air removed as possible in the freezer for up to 3 months. To serve from frozen, place on a plate and microwave for 30-60 seconds or until warm.