Preheat oven to 425 degrees. Spray a 9 x 9-inch baking pan with cooking spray.
Prepare the cauliflower into 1-inch florets. Add the florets to a pot of boiling water and cook, covered for about 5-8 minutes, until tender-crisp. Drain. Set aside.
Place onions in a microwave-safe small bowl and cook for about 1 minute in the microwave until soft. Melt butter in a medium non-stick pan. Add softened onions, garlic, and dried mustard. Sauté for 2 minutes. Stir continuously.
In a blender, add cooked onions, garlic, cottage cheese, buttermilk, and a little salt and pepper. Process until smooth. Add back to pan and mix in cooked cauliflower. Be sure to use a rubber spatula so as to not mash the cauliflower. Pour into baking pan and spread evenly. Sprinkle Parmesan cheese all over the top of casserole.
Bake until golden brown and sauce is bubbly, 35-40 minutes. Let gratin rest 10 minutes before slicing into 6 servings. This casserole will keep in the refrigerator for a few days and freezes great.