A flaky-crusted quiche layered with caramelized onions, sweet butternut squash, kale, and nutty Gruyere in a silky egg-and-milk custard. Comforting, make-ahead friendly, and perfect for vegetarian holiday brunch, lunch, or dinner.
Preheat oven to 350°F (177°C). Line the crust with parchment and fill with pie weights (or dried beans). Bake 20 minutes; remove parchment and weights, then bake until light golden, 5 to 10 minutes.
Melt butter in a skillet over medium-high heat. Add onions, 1/4 teaspoon of the salt, and 1/4 teaspoon of the pepper. Stir, cover, and cook 10 minutes, stirring occasionally.
Uncover, add the butternut squash, and cook until onions are golden and squash is tender, 8 to 10 minutes (deglaze with a splash of water if needed and reduce heat to avoid burning).
Add kale and cook until just softened, about 1 minute. Stir in thyme, remove from heat, and set aside.
In a medium bowl, whisk together eggs, milk, remaining 1/2 teaspoon salt, and remaining 1/4 teaspoon pepper.
Set the blind-baked crust on a baking sheet. Layer in the onion-squash-kale mixture and the cheese. Pour in the egg mixture to fill the crust (you may not use it all).
Bake at 350°F (177°C) until the custard is set but the center still has a slight wobble, 30 to 40 minutes. Let cool slightly before slicing and serving.