Stir together the blackberry jam, mustard, and rosemary in a small bowl until well combined. This step ensures that the flavors are evenly distributed, so each bite is as flavorful as the last. The mustard helps cut through the richness of the Brie, while the rosemary adds a fragrant touch.

Spread the mixture evenly on one side of a slice of bread. Make sure to cover the entire surface to prevent dry spots. Spreading evenly also ensures that the flavors meld together during cooking.

Layer the Brie slices on top of the spread and cover with the second slice of bread. Ensure the Brie is evenly distributed for optimal melting. Overlapping the slices slightly helps them melt into a creamy layer.

Coat a small nonstick skillet with cooking spray and heat over medium-low. The low heat will allow the cheese to melt fully without burning the bread. A lower temperature is key to achieving a perfect, golden crust.

Lightly spray the outsides of both bread slices with cooking spray. This helps achieve a golden, crispy crust without the excess calories of butter. If you prefer, you can use a thin spread of butter for extra flavor.

Place the sandwich in the skillet, cover, and cook until golden on the bottom (about 3 minutes). Covering the skillet traps heat and encourages even melting. If you notice the bread browning too quickly, reduce the heat slightly.

Flip the sandwich, cover again, and cook until the other side is golden and the Brie has melted (2–3 minutes). The melting point of Brie is low, so watch closely to avoid overcooking. Gently press the sandwich with a spatula to ensure even cooking.

Remove from the skillet, slice diagonally, and serve hot. A diagonal cut not only makes the sandwich easier to handle but also presents beautifully. Serve immediately to enjoy the gooey cheese at its best.