Heat a large non-stick or cast-iron skillet over medium-low heat to ensure the bread toasts evenly without burning.
Spread the ghee evenly onto one side of two sourdough slices, leaving the third slice plain.
Place one ghee-brushed slice into the skillet, ghee-side down, and spread the Dijon mustard across the top.
Layer half of the sharp cheddar cheese, half of the roasted turkey breast, and half of the baby spinach onto the first slice.
Place the plain slice of bread on top, then layer the remaining cheese, turkey, and spinach over it.
Top with the final sourdough slice, ghee-side facing up, and sprinkle the stack with sea salt and black pepper.
Cover the skillet with a lid for 2 to 3 minutes to allow the heat to penetrate the thick layers and melt the cheese.
Remove the lid, carefully flip the entire triple-decker stack with a large spatula, and grill until the bottom is a deep, crunchy golden brown.