This recipe is courtesy of Chuck at Sunday Nite Dinner, who is also the creator of Food Gawker, the best (imho) Tastespotting replacement out there. His picture is much better than mine, go look! (Sorry, I can't be bothered to figure out anything on my camera other than the Macro button. Maybe one of these days...)
You start by making a bread dough and caramelizing onions and leeks. A whole boatload of onions and leeks. So much of a boatload that my biggest skillet wasn't big enough, and my onions were steaming instead of caramelizing. So I removed the parchment paper and cranked up the heat a little, and all was well.
When the dough has risen and the onions are brown and sweet and happy, you stretch out the dough, spread it with mustard, spread the onions on top, and top with grated parmesan. Then bake for a while (25 minutes was enough for mine).
You end up with the best leek and onion tart you've ever tasted.