Cook pasta (12 ounces) according to package directions just until al dente. Drain and set aside.
While the pasta cooks, make sauce:
In a skillet, melt butter (2 tablespoons) then saute onion (1) until soft. Remove from heat and cool slightly.
In a large bowl, using a handheld mixer, combine softened cream cheese (8 ounces), condensed cream of chicken soup (2 cans), sour cream (2 cups), 1 cup of milk, 1 packet of ranch dressing mix, salt (1 teaspoon), and black pepper (1/2 teaspoon).
Using a large rubber spatula or spoon, fold in the cooked pasta, cooked onion, chicken (4 cups), 2 cups of cheddar cheese, and 1 cup of cooked bacon.
Pour into the prepared baking dish and spread evenly. Cover with foil and cook in preheated oven for 45 minutes.
Remove from oven, remove and discard the foil, sprinkle with remaining 2 cups cheddar and 3/4 cups remaining bacon, and return to oven. Continue to cook until casserole is heated through and bubbly and cheese are melted about 10-15 minutes. Remove from oven. Let cool for 10 minutes. Sprinkle with 3/4 cup green onions and serve.