Preheat the oven to 400ºF and line a large baking sheet with parchment paper.
FROZEN METHOD (fastest): If you're using frozen cauliflower rice (my preferred method) make sure you've thawed it in the fridge the night before. Frozen cauliflower florets can also be thawed ahead of time and used instead, you'll just need to "rice" it by processing it briefly in a food processor, before proceeding to the next step.
FRESH METHOD: If you're using fresh cauliflower, you'll need to steam it first. Fill a large pot with an inch of water, then place a steamer basket over that. Fill the basket with the fresh cauliflower florets and bring the water to a boil. Once boiling, cover the pot and lower the heat to a simmer. Cook until the cauliflower is very tender, about 10 to 15 minutes. Drain and transfer the cauliflower to a food processor. Process it briefly to make cauliflower "rice." Continue as directed, though you might want to let the cooked cauliflower cool so you don't burn your hands when wringing it out.
Transfer the thawed or cooked cauliflower rice to the center of a thin dish towel, or place it in a nut milk bag. Use your hands to squeeze the rice, removing all of the excess moisture from the cauliflower. There should be quite a lot of liquid that comes out, close to 1/2 cup.
Place the drained cauliflower in a large bowl, then add in ground flax or chia seeds, almond flour, salt, garlic and dried oregano. Stir well to mix, using your hands if needed to make it stick together. You can add a tablespoon of water if needed, but I typically don't add any extra liquid when I use my hands to mix this dough.
Press the mixture into the parchment-lined baking sheet, using your hands to shape the crust into your desired size. The thinner and flatter you can press the crust, the better, but try to keep it as even as possible for a sturdy result. I typically get a crust that is 12 to 14 inches in diameter from this recipe.
Bake the crust at 400ºF for 45 minutes, checking on it after 30 minutes to make sure it's not starting to burn. You want the crust to be dry to the touch and nicely golden. (Tip: You can roast vegetable toppings for your pizza at the same time! I put sliced tomatoes, red onion, and mushrooms on a separate pan, and take them out of the oven around the 25 minute mark so they don't burn.)
For best texture, I recommend flipping the entire pizza crust over on the pan (I use the parchment paper to lift it), then return it to the oven for another 10 to 15 minutes to dry out even more. However, this is optional. I do recommend flipping over the crust either way, so that the "dry side" is on the bottom, making it easier to pick up with your hands later.
Once the crust is firm and dry, add your favorite pizza toppings and return to the oven briefly to let everything heat up, about 5-10 additional minutes. Serve warm. Leftovers can be stored in the fridge for up to a week, or in the freeze for up to 3 months.
You can skip the cooking and cooling process when using frozen cauliflower. Simply allow the frozen cauliflower to thaw in your fridge overnight, which creates a "cooked" texture without having to do the extra work. Pulse the thawed cauliflower to create the rice, then drain well using a dish towel.