Preheat the oven to 375°F.
In a 12-inch skillet, cook the bacon over medium heat, stirring often until crispy, about 8 minutes. Remove the bacon from the pan using a slotted spoon. Set aside, leaving the drippings in the pan.
Add the onion and garlic to the drippings; cook, stirring often, until slightly softened, about 5 minutes.
Whisk in the cream cheese, milk, ranch seasoning mix, and 1 cup of the mozzarella until smooth; remove from the heat.
In a large bowl, stir together the ranch mixture, cooked pasta, chicken, pimientos, and ½ cup of the green onion until well combined.
Transfer to a 13-by-9-inch casserole dish. Sprinkle evenly with the bacon; top with the remaining 2 cups mozzarella.
Bake until the top is golden brown and crispy, 25 to 30 minutes.
Let cool slightly; top with more green onion just before serving.
