Heat a large non-stick skillet over medium-low heat and melt the ghee, spreading it evenly across the surface.
Spread a thin layer of Dijon mustard on one side of each of the three bread slices.
Place the first slice of bread in the skillet, mustard-side up, and layer with half of the cheddar cheese, half of the turkey, and half of the baby spinach.
Place the second slice of bread on top, mustard-side up, and repeat the layers with the remaining cheese, turkey, and spinach.
Top with the final slice of bread, mustard-side down, to close the triple-decker sandwich.
Cover the skillet with a lid for 2-3 minutes to ensure the thick stack heats through and the cheese melts thoroughly.
Carefully flip the sandwich using a wide spatula and grill the other side for an additional 2-3 minutes until deep golden brown.
Remove from the pan, season the exterior with a pinch of sea salt and black pepper, and let it rest for one minute before slicing.