Preheat oven to 200° C / 390° F. Line two baking trays with parchment or silicone baking mats, and set aside until ready to use.

Strawberry Jam:

Combine diced strawberries and sugar in a small saucepan. Simmer over low heat for about 15 minutes, or until berries begin to break down.

Add thyme leaves and simmer 5-10 minutes further, until thread stage* is reached.

Remove jam from heat and stir in lemon juice. Transfer to a bowl and set aside to cool completely.

Mascarpone Cream:

While jam is cooling, prepare the mascarpone cream. In a mixing bowl, combine mascarpone cheese, egg, lemon zest, lemon juice, and vanilla bean paste, and whisk until smooth and creamy. Set cream aside until ready to use.

Assembly:

Unroll pre-made puff pastry. Using a pastry cutter, cut pastry into 6-8 hearts* and discard any scraps.

Transfer hearts to prepared trays.

Spread a layer of mascarpone cream on each heart, allowing a 1/2 inch border around the edges. Top mascarpone with a spoonful of strawberry jam.

With a pastry brush, lightly brush the pastry border with egg wash, then fold the edges inward. Brush the folded edge with more egg wash.

Bake in preheated oven for 25 minutes, or until the pastry is flaky and golden brown. Allow pastries to cool. If desired, garnish with a sprinkling of fresh thyme leaves.

Serve immediately.