Heat a large non-stick skillet over medium-low heat to ensure even melting without burning the bread.
Lightly spread the grass-fed butter on one side of two bread slices, leaving the third slice plain.
Place one buttered slice, butter-side down, in the skillet.
Layer half of the sharp cheddar cheese, half of the turkey breast, and half of the baby spinach onto the bread.
Place the plain slice of bread on top, then layer the remaining cheese, turkey, and spinach.
Place the final buttered slice on top, butter-side up.
Season the exterior with garlic powder, sea salt, and black pepper.
Cover the skillet with a lid for 3 minutes to ensure the center is heated and the cheese is melted.
Remove the lid and grill until the bottom is golden brown, then carefully flip the entire sandwich.
Cook for another 2-3 minutes until the second side is crispy and the cheese is bubbling.
Remove from the pan, let it rest for 60 seconds to set, then slice diagonally and serve.