Preheat oven to 400 degrees. Line a baking sheet with parchment paper.

Working in batches as needed, pulse cauliflower in a food processor until reduced to the consistency of coarse breadcrumbs.

Transfer cauliflower crumbs to a large microwave-safe bowl; cover and microwave for 3 1/2 minutes.

Uncover and stir. Re-cover and microwave for another 3 1/2 minutes, or until hot and soft.

Transfer cauliflower crumbs to a fine-mesh strainer to drain. Let cool for 10 minutes, or until cool enough to handle.

Using a clean dish towel (or paper towels), firmly press out as much liquid as possible -- there will be a lot. Return cauliflower to the bowl, and add remaining crust ingredients. Mix thoroughly.

Divide crust mixture into two circles on the baking sheet, each about 1/4 inch thick and 7 inches in diameter.

Bake until the tops have browned, about 35 minutes.

Meanwhile, make the topping. Melt butter in a skillet over medium-high heat. Add onions, and sprinkle with salt. Stirring often, cook until softened and lightly browned, about 6 minutes.

Reduce heat to medium low. Add mushrooms. Stirring occasionally, cook until browned and caramelized, about 15 minutes.

Increase heat to medium high. Add spinach and garlic. Cook and stir until spinach has wilted, 1 - 2 minutes.

Divide topping mixture between the crusts. Top with mozzarella.

Bake until cheese has melted and crusts are crispy, 5 - 7 minutes.