Mosaic potato tart with watercress

Directions

1. Preheat the oven to 400F. On a lightly floured surface, roll out the puff pastry until it is approximately one square foot in area; the puff pastry should be pliable but still chilled. Gently score a 1" border around the edge with a sharp knife, and prick all over the center with a fork. Transfer to a baking sheet and keep in a cool place (like the refrigerator) until ready to use.
2. Peel the potatoes (try to get them into a roughly round shape) and slice very thinly with a mandoline. Use one of the smallest settings - you want the potato slices to be paper thin and translucent. Place the slices in a large bowl of cold water so that they lose some of their starch and do not turn brown. (Note: It is not possible to cut the potatoes thinly enough with a knife. If you do not have a mandoline, cut the potatoes in half and use a vegetable peeler to shave slices.)
3. Mince the garlic and place it in a heavy-bottomed pan with the olive oil. Over low heat, gently warm the olive oil until the garlic begins to sizzle. Turn the heat off and let the garlic sit for at least five minutes. While the garlic is resting, drain the potato slices and pat dry with paper towels.
4. Assemble the tart. Spread the crème fraîche onto the puff pastry, avoiding the border (an offset spatula is helpful here, but not necessary). Spread the mustard on top of the crème fraîche, and evenly distribute the garlic on top of the mustard. Starting with one side, arrange a row of potato slices so that they overlap slightly and form a scalloped edge. Arrange another row of potato slices overlapping the first row by 60-70%. Repeat until you have covered the entire tart (you may have extra potato slices). If potato slices curl, lay them concave side down. With a pastry brush, generously brush the top of the potatoes with heavy cream (you can also drizzle it very carefully with a small spoon, just try not to let it run off the side). Sprinkle the top of the tart with a few pinches of salt, keeping in mind that this is your only chance to season the potatoes. Sprinkle the Parmesan cheese sparingly over the top.
5. Bake until the pastry is golden and puffed and the potatoes show at least a faint hint of color here and there around the edges, usually about 30-35 minutes in all. Halfway through baking, take the tart out and brush or drizzle with heavy cream a second time (this moisturizes the potatoes and helps them cook). When the tart looks done, take it out, grind a bit of pepper over it, and artfully scatter some watercress leaves across the top. Return the tart to the oven for another two minutes, until the watercress is lightly wilted but still bright green. Serve warm.