1/2cupshredded cheese(gruyere, aged cheddar, or manchego work great for this)
Start by making the crust. In a large mixing bowl, combine the flour, sugar, and salt. Cut in the butter with a pastry cutter or two knives until the mixture resembles coarse crumbs. Whisk together the egg yolk and water, and add it to the flour mixture. Mix everything together with a fork until the dough comes together. Flatten into a disk, cover tightly with plastic wrap, and place in the refrigerator to chill for at least an hour. Take out of the refrigerator about a few minutes before you’re ready to roll it out.
Roll out the dough and fit it to your tart pan. You may have to patch it up a bit, but don’t worry, it’ll turn out great. Pierce the dough all over with a fork and place the tart shell in the freezer for 30 minutes.
Meanwhile, melt 2 tablespoons of butter in a large skillet over medium heat. Add the chopped leeks and cook for about 10 minutes. Add the kale, crushed red pepper flakes, and a couple drizzles of olive oil. Season with salt and pepper to taste. Cook for another couple minutes, just until the kale is wilted. Remove from the heat.
Preheat the oven to 375 degrees F (210 degrees C). Blind bake the tart shell for 10 minutes, and remove from the oven. If you see it starting to puff up at all, poke the dough with a fork again!
Prep your potato by peeling it and slicing it paper-thin. A mandolin helps, but a sharp knife will do the job. Add the thyme and ricotta (or cream cheese) to the kale mixture. Place a layer of sweet potato slices in the bottom of the tart shell. Spread the kale and cheese mixture evenly over that, and top with another layer of sweet potato slices. Brush the top thoroughly with olive oil and sprinkle the cheese over the tart. Bake for about 20 minutes, until golden brown.