Preheat your oven to 400°F and have a baking sheet at the ready. Place a sheet of parchment paper on your work surface.
Unfold the puff pastry dough directly on the parchment. Gently roll the dough to a 9x13 rectangle. It needn't be exact. Transfer the parchment and dough to the baking sheet.
1 frozen puff pastry
To create a border for your tart, run a dull knife around the edge of the rectangle, making sure to stay about an inch away from the edge. However, make sure you don't cut all the way through the dough, just score it. The resulting frame will give your tart a nice border once it has been baked and gives you a 'field' to assemble your ingredients.
Use the back of a spoon to spread the Dijon inside the field, side to side, corner to corner. Crumble the Boursin cheese inside the field over the Dijon, again side to side, corner to corner. Working within the field, arrange the asparagus spears.
1 tablespoon Dijon mustard, 1 5.2-ounce box Boursin Shallot and Chive cheese, 8 ounces thin asparagus stalks
Randomly dot the slices of brie on top the asparagus. You can slice and leave the edible rind remaining or remove it. For the purposes of my photos, I have removed it. Use a pastry brush to LIGHTLY apply the sour cream all around the exposed edges.
2 ounces brie, 1 tablespoon sour cream
Bake in the fully preheated oven for 15-18 minutes, or until puffed and golden. Remove, scatter fresh thyme over the top, and place back into the oven for an additional 2 minutes. You want the thyme exposed to the high heat only briefly, so it releases it oils and smells perfectly earthy.
thyme leaves