Preheat a non-stick skillet or griddle over medium-low heat to ensure even melting without burning the bread.
Spread the light butter evenly across one side of each bread slice.
Spread Dijon mustard on the non-buttered side of one slice.
Place one slice of bread, butter-side down, onto the skillet.
Layer one slice of cheese, followed by the turkey breast, then the baby spinach.
Top with the second slice of cheese and the remaining bread slice, butter-side up.
Cook for approximately 3-4 minutes until the bottom slice is golden brown and the cheese starts to soften.
Flip the sandwich carefully using a spatula and cook for an additional 3-4 minutes until the second side is toasted and the cheese is fully melted.
Remove from heat and allow to rest for 60 seconds before slicing diagonally.