I just threw this together and it turned out fantastic. The reason I recommend that the bacon be added to the top only, is because it loses its crispiness if mixed in.
Peel, quarter, boil & drain potatoes.Preheat oven to 350 degrees. Place potatoes in a large mixing bowl. Mash with potato masher.Mix in 2 cups of cheese while still hot.
In a separate bowl, combine sour cream, milk, & ranch seasoning. Mix well. Add mixture to potato mixture and blend together with 3/4 cup green onion.
Pour into casserole dish and top with bacon and remaining green onion & cheese. Bake for 25 minutes.