Preheat oven to 400°F (200°C). Line a baking sheet with parchment paper.

In a medium bowl, gently toss together the cranberries, red onion, Dijon mustard, olive oil, thyme, salt, and pepper.

Unfold the puff pastry on a lightly floured surface. Using a 3-inch cookie cutter or a knife, cut out 6 circles.

Place a puff pastry circle in each cup of a muffin tin. Gently press the pastry into the bottom and up the sides of each cup.

Divide the cranberry mixture evenly among the tartlet shells. Top each with a few pieces of brie cheese. (Optional: Brush the edges of the pastry with the beaten egg for a richer golden color).

Bake for 15-20 minutes, or until the pastry is golden brown and the cheese is melted and bubbly.

Let the tartlets cool in the muffin tin for a few minutes before carefully removing them to a wire rack to cool completely.

Serve warm or at room temperature.