Preheat Oven: Set oven to 400°F (200°C). Grease a 9×13-inch casserole dish.
Parboil Potatoes: Place diced potatoes in a large pot with salted water. Bring to a boil and cook 8–12 minutes, until potatoes are tender but still firm. Drain well.
Toss with Ranch: While warm, toss the potatoes with ranch dressing, butter, garlic salt, and black pepper until evenly coated.
Bake Potatoes: Transfer the potatoes to the prepared casserole dish. Bake uncovered for 20–25 minutes, until lightly golden.
Add Cheese: Remove the casserole from the oven and sprinkle the shredded cheese over the top. Return to the oven and broil for 2–3 minutes, or until the cheese is melted and bubbly.
Serve: Garnish with chopped chives or green onions if desired. Serve immediately while warm.