Heat 1 tablespoon of the oil in a
large cast iron skillet over medium heat. Add the onion,
season with some salt and pepper, and cook until golden,
about 7 minutes. Put in a bowl and set aside. Wipe out
the skillet and add another tablespoon of oil. Season
the turkey patties with salt and pepper, and cook in the
remaining oil until well browned, about 3 minutes per
side. Remove from the skillet. Wipe out the pan.
Divide the onions among 4 bread slices, and top with
a slice of cheese, a patty, another slice of cheese, and
a slice of bread. Spread the top and bottom of the
sandwiches with some butter. Heat the cast iron skillet
over medium heat. Cook the sandwiches, in batches if
needed, turning once, until lightly browned and the
cheese melts, about 2 minutes per side. Halve the
sandwiches and serve with cranberry sauce, if desired.