Spoon small portions of mustard onto the parchment, spacing them apart to form the base of each tart

Arrange thin apple slices over the mustard in slightly overlapping layers, creating a neat, fan-like pattern

Place slices of brie on top of the apples and sprinkle fresh thyme for a fragrant herbal note

Roll out the puff pastry lightly and cut into equal squares that will fully cover each apple stack

Gently place the pastry over the filling, pressing the edges down so it seals around the apples and cheese

Brush the tops with beaten egg until glossy for a golden finish

Bake until the pastry is puffed and deeply golden, with a crisp exterior and bubbling edges

Let the tarts rest briefly, then carefully flip them over to reveal the caramelized apple topping