Preheat the oven to 350°F. Grease a nonstick baking sheet with cooking spray.
Bring a large pot of salted water to a boil. Add the broccoli florets to the water and cook them just until fork tender, about 5 minutes. Thoroughly drain the florets and transfer them to a food processor. Pulse the broccoli for a few seconds just until the it breaks down into small pieces. (Do not overmix the broccoli or the mixture will be too wet to form into tots.)
Measure out 3 packed cups of the broccoli and add it to a large bowl. Add the diced onion, breadcrumbs, egg and Parmesan cheese and mix until thoroughly combined.
Using your hands, portion out about 2 tablespoons of the mixture and mold it into a tater tot shape. Arrange the tots on the prepared baking sheet, spacing them about 1 inch apart.
Bake the tots for about 20 minutes then flip them once and bake them an additional 10 to 15 minutes until crisped. Remove the tots from the oven and serve them with ketchup, Ranch dressing or hummus for dipping.