Preheat oven to 500°.
Heat oil in a large skillet over medium heat. Add onions and garlic; cook 5 minutes or until browned, stirring frequently. Remove from heat.
Place onion mixture and cauliflower in a roasting pan coated with cooking spray. Combine water and mustard; pour over vegetable mixture. Toss to coat; sprinkle with salt and pepper. Bake at 500° for 20 minutes or until golden brown, stirring occasionally. Sprinkle with parsley.