In a skillet over medium heat, melt the butter. Add the leeks, and cook until softened, about 5 minutes.
Add the chicken stock and lower the heat. Cook for about 10 minutes, or until softened and caramelized. Add the lemon juice, salt and pepper. Set aside.
In a large bowl, combine the turkey, breadcrumbs, herbs, sour cream, garlic powder, salt and pepper. Form into 4 thin patties.
In a skillet over medium high heat with a bit of oil, cook the turkey patties about 3-4 minutes per side, or until cooked through.
Brush the bread with mustard. Place a slice of swiss cheese on the bottom pieces. Top with the turkey patties, American cheese, and caramelized leeks. Brush the outsides of bread with butter.
Cook the sandwiches in skillet until golden brown and melted, about 2-3 minutes per side.
