In a medium saucepan over medium heat, combine fresh diced strawberries, balsamic vinegar, granulated sugar, lemon juice, and black pepper. Stir well and bring to a simmer. Cook for 20-25 minutes until thickened.

Remove the jam from heat and allow it to cool completely. This step is essential for the right spreadable texture.

Spread butter evenly on one side of each slice of bread.

Lay half of the buttered bread slices in a skillet, butter-side down. Spread cooled jam on unbuttered sides, add brie slices, then top with remaining buttered bread slices.

Preheat skillet over medium-low heat. Grill sandwiches for 3-4 minutes per side, pressing down gently.

Transfer grilled sandwiches to a cutting board and let them rest for a minute before slicing. Garnish with fresh basil if desired.