I use a frozen deep pie crust. Set out for 20 minutes, poke with a fork and bake 10 minutes.
Saute onions in butter until they cut easily.
Cobine milk, eggs, salt and 1 cup sour cream and mix lightly.
Blend flour with remaining 1/2 cup of sour cream and add to egg mixture (will be lumpy).
Place cooked onions in pie crust and pour liquid mixture over onions and lightly stir in.
Combine cheddar cheese and mayo, mix well.
Spoon on top of pie in dollops.
Bake at 350 degrees for 40-45 minutes or until it browns.