Preheat the oven to 450 degrees F (230 degrees C). Line a baking sheet with parchment paper.
Whisk 1/4 cup water and flaxseed meal together in a large bowl. Let sit for 5 minutes.
Place a steamer insert into a saucepan and fill with water to just below the bottom of the steamer. Bring water to a boil. Add cauliflower, cover, and steam until fall-apart tender, 3 to 6 minutes. Let cool, at least 10 minutes.
Grind oats, rosemary, 1/2 teaspoon salt, and 1/4 teaspoon garlic powder together in a food processor until flourlike in texture.
Place cooled cauliflower in a fine cheesecloth and squeeze out as much moisture as possible. Add pressed cauliflower to the bowl with the flaxseed-water mixture and stir well. Stir in the oat mixture. Form dough into a ball and place onto the lined baking sheet.
Press dough ball down into a circle. Lay a sheet of parchment on top and roll dough flat using a rolling pin to about 1/4-inch thick.
Bake in the preheated oven until crust is set, about 15 minutes. Remove from the oven; leave oven on.
Place cashews into a pot with water to cover; bring to a boil. Reduce heat to medium-low and simmer until soft, about 15 minutes. Drain.
Puree boiled cashews, 1 1/4 cups fresh water, tapioca starch, nutritional yeast, cider vinegar, 1/2 teaspoon salt, and 1/4 teaspoon garlic powder in a blender. Pour puree into a saucepan over medium heat. Cook, stirring continuously, until thickened, about 5 minutes.
Heat olive oil in a saucepan over medium-high heat. Saute garlic until fragrant, about 2 minutes. Add crushed tomatoes, balsamic vinegar, maple syrup, 1 teaspoon salt, red pepper flakes, oregano, and black pepper. Simmer marinara sauce until flavors blend, about 10 minutes.
Fill blender halfway with marinara sauce. Cover and hold lid down with a potholder; pulse a few times before leaving on to blend. Pour into a bowl. Repeat with any remaining marinara sauce.
Spread marinara sauce over the cauliflower crust; top with the 'mozzarella cheese' and add cherry tomatoes.
Bake in the hot oven until sauce is bubbling and crust is golden brown, about 8 minutes. Top with basil.