Preheat the oven to 350°F. Grease a nonstick baking sheet liberally with cooking spray.
Bring a large pot of salted water to a boil. Add the cauliflower florets to the water and cook them just until fork tender, 5 to 10 minutes. Thoroughly drain the florets and transfer them to a food processor. Pulse the cauliflower for a few seconds just until the it breaks down into small pieces (about the size of grains of rice). (Do not overmix the cauliflower or the mixture will be too wet to form.)
Measure out 3 packed cups of the cauliflower and add it to a large bowl. (See Kelly's Notes.) Stir in the diced onion, Parmesan cheese, breadcrumbs, egg, 1 teaspoon salt and 1/4 teaspoon pepper, mixing until thoroughly combined. (The mixture should be roughly the consistency of mashed potatoes.) Using your hands, scoop up 1- to 2-tablespoons of the mixture and mold it into a tater tot shape. Place it on the prepared baking sheet and repeat the shaping process with the remainder of the mixture, spacing the tots about 1 inch apart.
Bake the tots for about 20 minutes then flip them and bake an additional 10 to 15 minutes until crisped. Remove and serve with ketchup, pesto, hummus or your preferred dipping sauce.