Have the dough prepared and chilled. Position a rack in the lower third of the oven (this will help crisp up the bottom crust) and preheat to 400ºF.
Bring a large saucepan of salted water to a boil. Peel the sweet potatoes, trim away the narrow ends, and slice them ¼” thick. Carefully place in the boiling water and simmer gently until they’re just fork tender but not falling apart, 3-5 minutes; take care not to overcook. Drain and rinse with cool water to stop the cooking. Set aside.
Strip the kale leaves from the ribs, discard the ribs, and chop the greens fairly finely into roughly 1-inch squares. Heat the oil in a wide skillet over medium heat and when it shimmers, add the onion and garlic. Cook, stirring frequently, until the onion is tender and golden, 5-10 minutes. Add the kale, a splash of water, ¼ teaspoon salt, and a few turns of pepper. Cook, stirring frequently, until the kale is wilted and bright green, 5-10 minutes. Remove from the heat and let cool.
In a measuring pitcher, whisk together the cream, egg, ¼ teaspoon salt, and a few turns of pepper until well-combined. Chill until needed.
Meanwhile, shape the dough. If the dough is cold, let it stand at room temperature until slightly softened, 5 minutes in a warm kitchen or 15 minutes in a cool kitchen. Sandwich the dough between two pieces of parchment paper dusted lightly with oat flour, and gently begin pressing it flat, then rolling it into a 10×15-inch rectangle. As you work, periodically peel back the top piece of parchment, dust the dough lightly with flour, replace the parchment, grasp the dough sandwich with both hands, and flip the whole thing over. Peel off the new top piece of parchment, dust with flour, and continue to roll. If the dough cracks or tears, just squish it back together. Use a pastry brush to sweep away excess flour from the dough. If the dough is uneven, cut off the long bits and press them onto the short bits, rolling to adhere. When your rectangle is 10×15 inches, trim the sides so that they’re even and straight. (If your dough becomes soft or sticky at any point, slip it onto a baking sheet, parchment and all, and chill it for 10-20 minutes to firm the butter.)
Slide the dough onto a (preferably rimless) sheet pan, still on the parchment. Sprinkle the grated cheese evenly over the dough, leaving a 1″ border all the way around. Top with the cooked kale. Lay the parboiled sweet potato slices over the kale, overlapping in rows, until you’ve covered the whole thing; you may not need all the sweet potato slices. Gently fold over the edges to make a crust, pressing the corners to adhere. Brush the sweet potatoes generously with olive oil, and sprinkle liberally with salt. Brush the edges of the crust with a little of the custard mixture, then slowly pour the remaining custard over and around the sweet potatoes; it will sink through to the bottom. Top with a few turns of pepper and put it in the oven right away.
Bake the tart at 400ºF until the edges are deeply bronzed, 40-50 minutes. Don’t underbake or the bottom crust will be soggy. Remove from the oven, carefully slide onto a large serving board, cut into wedges, and serve.
Leftover tart keeps well, refrigerated airtight, for up to three days. I like to reheat slices in a cast iron skillet over very low heat; by the time the top is heated through, the bottom crust is extra crisp.