1. Cook bacon in a large nonstick skillet over medium heat until crisp. Remove bacon from pan; crumble. Increase heat to medium-high. Add onion, mushrooms, and garlic to drippings in pan; sauté for 6 minutes. Add potatoes and stock; cook 6 minutes, stirring frequently. Add spinach, basil, 1/4 teaspoon salt, and 1/4 teaspoon pepper; cook 2 minutes or until spinach wilts. Remove from heat; let stand 10 minutes. Stir in crumbled bacon and cheese. Place mushroom mixture in an 11 x 7-inch broiler-safe glass or ceramic baking dish coated with cooking spray. Cover and refrigerate overnight.
2. Preheat oven to 350°.
3. Uncover dish. Combine remaining 1/4 teaspoon salt, 1/4 teaspoon pepper, milk, and eggs in a medium bowl. Pour egg mixture over mushroom mixture. Bake at 350° for 28 minutes.
4. Preheat broiler to high; remove dish while broiler preheats. Broil 3 minutes or until top is browned and just set. Let stand 5 minutes.