In a small bowl, whisk together the Greek yogurt, Dijon mustard, sea salt, and black pepper to create a creamy, protein-rich spread.
Lay out the three slices of sprouted grain bread and spread the yogurt mixture evenly across one side of each slice.
On the first slice, layer half of the cheddar cheese, half of the roasted turkey, and half of the fresh spinach.
Place the second slice of bread on top, spread-side up, and repeat the layering with the remaining cheese, turkey, and spinach.
Top with the final slice of bread, spread-side down, to complete the triple-decker stack.
Heat the ghee in a large cast-iron skillet or non-stick pan over medium-low heat until melted and shimmering.
Carefully place the sandwich in the skillet and cook for 3-4 minutes, pressing down gently with a spatula to ensure even browning.
Flip the sandwich carefully and grill for another 3 minutes until the bread is golden brown and the cheese is thoroughly melted.