Preheat your oven to 350°F. Lightly grease a 9x13-inch baking dish. In a medium skillet, heat the olive oil. Add the onion and cook until softened, then add the garlic and cook 1 more minute. Remove from the heat.

In a large bowl, whisk together the sour cream, cream of chicken soup, milk, cheddar cheese, ranch seasoning mix, salt, and pepper. Stir in the shredded chicken, onion, and garlic.

Bake in the preheated oven for 35-45 minutes until bubbly and golden. Let cool for 5 minutes before serving. Garnish with green onions.