1½ pounds sweet potatoes (about 4 small), peeled, cut into ½-inch or smaller pieces
1 large onion, thinly sliced
12 large eggs
4 ounces sharp cheddar, grated (about 1 cup)
1 cup whole-milk Greek yogurt
Freshly ground black pepper
Preheat oven to 300°.
Heat 2 tablespoons of oil in a large nonstick skillet over medium. Add potatoes, season with salt, and cook, tossing occasionally, until well browned around the edges and just slightly undercooked, 10–12 minutes.
Spray inside of springform pan with non-stick cooking spray. Transfer potatoes to springform pan and let cool.
In same skillet, heat remaining 3 tablespoons oil over medium. Cook onion, tossing occasionally, until softened but not browned, 8–10 minutes. Add kale a handful at a time and cook, stirring, until just softened but not limp, 5–6 minutes. Remove wilted kale, add more, repeat until all kale is cooked. Season with salt. Remove from skillet and let cool slightly.
Whisk eggs, cheese, and yogurt in a large bowl; season with salt and pepper. Set aside.
Place your springform pan on a rimmed baking sheet. Flatten potatoes slightly with the back of a spoon, packing into the seam where 2 parts of springform pan meet.
Top potatoes with half of kale and onions, then pour in egg mixture. Gently add remaining kale and onions atop the surface of egg mixture.
Bake quiche until edges have puffed up slightly and top is just set with no liquid egg remaining, 55–75 minutes. If quiche is beginning to brown at edges before completely set, cover with foil and continue to bake.
Let cool before slicing. Serve warm or at room temperate.