Line up 8 slices of bread on a work surface. Place a griddle or large skillet over medium heat.
In a small mixing bowl, combine the sour cream, ketchup and relish. Set aside.
Build 4 sandwiches with 2 or 3 slices of turkey, 2 slices of cheese, a few tablespoons of sauerkraut and some sauce per sandwich. Cap off each sandwich with the remaining bread slices and dip them all in the beaten egg (just like French toast). Add butter to the griddle and cook sandwiches 4 minutes on each side, turning with thin spatula.