In a large skillet, heat the butter and oil, add the onions and cook over medium heat until onions are just limp. Add the potatoes, salt, pepper and thyme. Cook 3 to 4 minutes, stirring often until potatoes are hot. Add milk, reduce heat to medium-low, cover and simmer stirring at least once, until potatoes are tender and most of the milk is absorbed, about 10 minutes. Remove from heat, stir in the scallions. Fill pie shell with filling and place in preheated 425 degree oven, bake 25 to 30 minutes or until golden brown. Cut into wedges to serve.

This can be made the day before it is to be served. Simply cool the filling before filling the pie shell, cover and refridgerate. Then bake as directed, but add about 5 minutes to the cooking time.