Preheat your oven to 475°F (245°C) for a standard oven, or 425°F (220°C) for convection.
In a small bowl, stir the yeast into the warm water until mostly dissolved. Let it sit for 5 minutes until foamy.
In the bowl of a stand mixer, combine the gluten-free flour, dry milk, sugar, and salt. Mix on low speed with the whisk attachment for 4 minutes.
Add the foamy yeast mixture and vegetable oil to the dry ingredients. Mix on medium speed for 10 minutes.
Generously oil a half-sheet pan and sprinkle it evenly with the cornmeal.
Pour the batter into the prepared pan and spread it evenly to the edges. Bake for 10 minutes in a standard oven.
While the crust bakes, whisk together the dried onion, garlic powder, pepper, basil, oregano, marjoram, thyme, tomato paste, and water in a saucepan. Bring to a simmer over medium heat, then cook for 10 minutes.
Keep the oven at 475°F (standard) or bump to 450°F (convection). Sprinkle 1½ cups of the shredded mozzarella evenly over the hot parbaked crust. Dollop and spread the sauce on top.
Scatter the remaining cheese (and cooked ground beef, if using) all the way to the pan's edges. Bake 10–15 minutes more, until the top is bubbly and golden-brown.
Let it cool 5 minutes in the pan. Slice into squares or rectangles.
