This 5-ingredient tart is perfect for a simple dessert or a holiday treat.
Defrost the frozen puff pastry. Preheat the oven to 400 degrees F.
Roll out the puff pastry (still chilled) on a well-floured pastry mat or board. Gently roll it close to a 12-inch square. (No need to be precise.) Line an 8-9 inch tart pan, folding and crimping the edges of the pastry with your fingers. Freeze the dough in the pan for about 5 minutes.
Mix the grapes, sugar, and lemon juice in a bowl. Let them sit for a few minutes, until the sugar begins to dissolve.
Place the grapes on the chilled tart dough in a single layer, then scrape the rest of the sugar and lemon juice mixture on top. Bake for 35-45 minutes, checking after about 20 minutes to make sure the pie crust edges are not getting overly brown. If the crust is browning too quickly (before the tart is done), put foil around the edges of the tart to prevent the crust from burning.
Once it is done, remove the tart from the oven and sprinkle the diced rosemary on top. Let the tart cool down to room temperature before serving.