in a large skillet, melt 1 tbsp. butter over medium heat. Add the onions, garlic and sage. Cook, stirring, until the onions are tender, 20 minutes. Stir in the gravy, if using. Season; cover to keep warm.
Spread one side of the bread slices with the remaining butter. Turn the bread over. Spread 4 slices with the cranberry sauce, then top with half the cheese, the turkey, the onion mixture and remaining cheese. Spread the remaining 4 slices with mayo and use them to close the sandwiches, mayo side down.
Heat a large skillet over medium. Add the sandwiches; cover and cook until the bread is golden and the cheese melts, 4 to 5 minutes per side.