Bring 1 3/4 cups water, 2 cups stock, 1 cup milk, and 1/2 teaspoon salt to a boil in a large saucepan. Reduce heat to medium-low; stir in polenta. Cook 20 minutes or until thick, stirring frequently. Pour 3 cups polenta into an 8-inch square baking dish coated with cooking spray; smooth surface. Cool, cover, and refrigerate. Stir 1/4 cup water, remaining 2 tablespoons milk, butter, black pepper, and 2 tablespoons Parmesan cheese into remaining 2 cups polenta; keep warm.
Preheat oven to 450°.
Coat a jelly-roll pan with cooking spray. Combine remaining Parmesan, rind, onion powder, and ground red pepper in a dish. Combine remaining 2 teaspoons water and egg in a dish. Dip cauliflower in egg mixture; dredge in Parmesan mixture. Arrange cauliflower on prepared pan; coat with cooking spray. Bake at 450° for 15 minutes, stirring after 6 minutes. Place cauliflower, parsley, juice, and remaining 1/4 teaspoon salt in a bowl; toss.
Heat oil in a skillet over medium heat. Add mushrooms; sauté 7 minutes. Add remaining 2 tablespoons stock and vinegar to pan; cook 2 minutes. Place 1/2 cup polenta in each of 4 bowls; top with 1 cup cauliflower mixture, 1/4 cup mushroom mixture, and hazelnuts.