In a separate bowl, mix whipping cream, sour cream and shredded cheese. Add mushroom and onion mixture. Gently add cauliflower and mix until blended well. Spray a 9″ x 9″ baking dish with cooking spray and pour in cauliflower mixture. Bake at 350 degrees for about 35 minutes until golden brown. If you like it crispy on top, you can broil it for about 2 minutes – just be sure to watch it closely.