For the tart crust, refer to my puff pastry recipe. Once crust is made, roll out into you desired shape (I used a large heart-shaped cookie cutter) at 1/4-1/2" thickness. Don't have a cookie cutter? No problem! Try turning over and cutting the circumference of a small bowl with a knife to trim create the tart.
Create a border around the perimeter of the pastry by rolling excess dough into a rope-like formation. Creating a "border" will keep the vegetables neatly inside the tart.
Finely chop leeks and slice Gruyère cheese into slices at any desired thickness. Set cheese aside.
Braise chopped leeks in a pan over medium-high heat with olive oil.
Place pastry shapes on a parchment lined baking sheet.
With the gruyere slices create a layer of cheese at the base of the tart.
Generously assort braised leeks on top of the tart and Gruyère base.
Optionally sprinkle each tart with white truffle oil, fleur de sel (or salt of choice), and cracked black pepper.
Bake for 10-15 minutes or until tarts are golden brown.