Preheat your oven to 190°C. This temperature ensures the baguette slices toast evenly while the Brie cheese melts to perfection. Preheating is essential for consistent cooking results.

Take a fresh French baguette and cut it into thin slices, about 1 cm thick. Slicing the bread thinly gives a crispy texture, which balances beautifully with the creamy Brie and rich toppings. Too thick, and it might overwhelm the delicate flavors; too thin, and it may burn quickly.

Spread a generous layer of caramelised onions on each baguette slice. The onions bring a sweet and savory depth that complements the creaminess of the Brie and the salty prosciutto. Make sure to cover the surface evenly to distribute the flavor with every bite.

Place a slice of Brie cheese on top of the caramelised onions. Try to cover as much of the bread’s surface as possible, ensuring the cheese melts across the entire slice. Brie’s creamy texture enhances the richness of the crostini once melted.

Arrange the prepared slices on a non-stick baking sheet or a tray lined with baking paper. This prevents sticking and makes cleanup easier. Bake the crostini in the preheated oven for 8-10 minutes, keeping a close watch. You want the cheese fully melted and the bread lightly toasted but not burnt.

Remove the tray from the oven and allow the crostini to cool slightly. Carefully drape a ribbon of prosciutto on each warm slice. The residual heat softens the prosciutto, intensifying its savory flavor and creating a delightful contrast with the creamy Brie.

Finally, top each crostini with a small sprig of fresh thyme. This subtle garnish adds a refreshing aroma and a hint of earthy flavor. Serve the crostini warm to enjoy the perfect blend of textures and flavors—crispy bread, creamy Brie, sweet onions, and savory prosciutto. Your guests will be delighted with this elegant, yet easy-to-make appetizer.