BUTTER the outside of each slice of bread.
DRIZZLE balsamic reduction over strawberries. Set aside.
SPREAD mascarpone on the inside of one slice of sourdough.
LAYER balsamic strawberries on top of mascarpone.
TOP with final slice of bread.
HEAT a pan on medium-low heat.
FLIP the sandwich when bread starts to turn golden brown and cheese has begun to melt.
REMOVE from the pan once both sides of bread are toasted and the cheese is fully melted.