Peel and dice your potatoes into bite-sized cubes. Place them in a large mixing bowl, drizzle with olive oil, and season with garlic powder, onion powder, salt, and pepper. Toss well to coat evenly.
Add the cup of ranch dressing and half the shredded cheddar cheese to the seasoned potatoes. Mix everything together until the potatoes are thoroughly coated.
Lightly grease your 9x13-inch baking dish. Spread the potato mixture evenly across the bottom. Lay the boneless, skinless chicken breasts on top of the potatoes. Pour the ½ cup of milk over the chicken and potatoes to keep everything moist during baking.
Sprinkle the remaining cheddar cheese generously over the chicken breasts. This will melt into a delicious, golden crust as the casserole bakes.
Cover the baking dish tightly with aluminum foil. Bake at 375°F (190°C) for 45 minutes. This allows the chicken to cook through and the potatoes to soften.
Remove the foil and bake for an additional 15-20 minutes until the cheese is bubbly and golden brown and the chicken reaches an internal temperature of 165°F (74°C).
Once out of the oven, sprinkle the chopped green onions over the casserole for a fresh, vibrant finish. Let it rest for 5 minutes before serving.
