BUTTER the outside of each piece of bread.
SPREAD pesto on the inside of one slice of bread.
LAYER havarti, spinach and turkey (if using) on top of each other.
TOP with final slice of bread.
HEAT a pan on medium-low heat.
PLACE your sandwich into the pan.
FLIP the sandwich when bread starts to turn golden brown and cheese has begun to melt.
REMOVE from the pan once both sides of your bread are toasted and the cheese is fully melted.