Loaded Mashed Potato Casserole

Name: Loaded Mashed Potato Casserole
Ingredients: 2 pounds Yukon Gold potatoes , scrubbed and chopped (about 6 cups ); 1 cup shredded sharp Cheddar cheese , divided; 4 ounces reduced-fat cream cheese , softened; 1/3 cup sliced scallions , divided; 1/4 cup reduced-fat milk; 1/4 cup reduced-fat sour cream , divided; 1 large egg , lightly beaten; 1 teaspoon salt-free garlic-and-herb seasoning (such as Mrs. Dash); 1/2 teaspoon salt; 1/4 teaspoon pepper; 2 tablespoons water; 3 slices bacon , cooked and crumbled