Preheat the oven to 375°F (190°C).

In a large pot, bring salted water to a boil and cook the diced potatoes until tender, about 10–12 minutes. Drain and set aside.

While the potatoes are cooking, heat olive oil in a skillet over medium heat and sauté the shredded chicken until heated through. Set aside.

In a large mixing bowl, combine the cooked potatoes, shredded chicken, ranch seasoning, sour cream, milk, cheddar cheese, bacon bits, and half of the green onions. Stir to combine and season with salt and pepper.

Transfer the mixture to a greased 9×13-inch casserole dish. Spread evenly and top with mozzarella cheese (if using).

Bake for 20–25 minutes, until the casserole is hot and bubbly, and the cheese is melted and golden.

Remove from the oven and sprinkle the remaining green onions over the top for garnish.

Serve hot and enjoy!