Place 1 slice each of cheese and turkey on each of 4 bread slices.
In pie plate beat eggs, milk, and dry soup mix until well blended.
Dip each sandwich into egg mixture,
spooning onion pieces onto bread.
Make sure all egg mixture is used.
Place butter in 10x15" inch jelly roll pan.
Set in preheated 450 degree oven a couple of minutes to melt butter.

Carefully place sandwiches in pan
and drizzle any remaining egg mixture over them. Bake 5 minutes.
Carefully turn sandwiches and continue baking until golden brown.
Serve with dijon mustard dipping sauce.

Dijon mustard dipping sauce:
1/2 cup sour cream
2 tbsp. milk
1 tbsp. dijon mustard

Mix well and then chill until ready to serve.