HEAT oven to 375ºF.
PAT hash browns dry with paper towels; place in bowl. Add butter; mix lightly. Press onto bottom and up side of 9-inch pie plate.
BAKE 20 min. or until golden brown. Meanwhile, whisk eggs and milk in medium bowl until blended. Stir in sour cream, then cheese and onions.
REDUCE oven temperature to 350ºF. Pour filling into crust. Bake 30 min. or until center is set and crust is golden brown. Let stand 5 min. before serving.