Combine the water, non-dairy milk, and granulated onion in a medium saucepan. Bring to just about boiling over medium-high heat. Gradually add in the cornmeal, stirring constantly with a whisk to prevent clumping. As soon as you start adding the cornmeal, turn heat to the lowest setting, and cook covered for 20 minutes, stirring two or three times throughout. Stir in the rosemary at the very end. While the polenta is cooking, you can make the sauce (or you can also make the sauce ahead of time, to allow the flavors time to mingle).
Scrape about half of the cooked polenta into your pan (it will be very thick and sticky), adding more as you go to reach a thickness of about a quarter inch, spreading it evenly with your fingers, and pressing it up the sides of the pan for a more deep-dish effect. If it's too hot to handle, let it cool for a couple minutes; you may also use dish gloves or a large piece of parchment paper to avoid sticky fingers as you are pressing it, or wet your fingers a couple times throughout. For my 14-inch pizza pan, I use about ¾ of the polenta. For a thicker crust or bigger pan, use more. Set aside.