Whisk together 1 (1-oz.) package Ranch dressing mix, 1 cup sour cream, and 1 3/4 cups milk in a large glass bowl. Stir in 1 (22-oz.) package frozen mashed potatoes. Microwave on HIGH 12 minutes, stirring every 4 minutes. Stir in 6 cooked and crumbled bacon slices and 1/2 cup shredded Cheddar cheese. Garnish with sliced green onions, if desired.