Casserole, Portobello Layered Mashed Potatoes

Ingredients

• 3 pounds Yukon Gold potatoes
• 3/4 cup low-fat buttermilk
• 1 teaspoon salt, divided
• 1/4 teaspoon ground nutmeg
• 1/4 teaspoon pepper, divided
• 1 1/2 teaspoons butter or stick margarine
• 1/4 cup minced fresh onion
• 2 garlic cloves, minced
• 1 pound finely chopped Portobello mushroom caps, about 3-1/2 cups
• 1/3 cup chopped fresh basil, or 2 tsp. dried
• cooking spray
• 1 ounce grated fresh parmesan cheese, 1/4 cup
• 1/2 teaspoon paprika
• 1/2 teaspoon olive oil