1 stick cold unsalted butter, cubed
1 ½ cups all-purpose flour
¼ teaspoon kosher salt
1 teaspoon granulated sugar
4-5 tablespoons ice water
3 cups fresh spinach, stems removed
½ yellow onion, chopped
5 large eggs
2 cups half-and-half
1 ½ cups 3-cheese blend (option: cheddar, Monterey Jack, mozzarella)
½ teaspoon garlic powder
Mix flour, salt, and sugar together in a food processor. Add cubed butter and pulse until butter is about pea-sized. Slowly add 3 tablespoons of ice water and pulse. Add 1-2 more tablespoons as needed until dough comes together, pulsing after adding each tablespoon.
Remove dough from food processor and roll into an oval, then flatten. Wrap in plastic wrap and chill for 20-30 minutes.
Preheat oven to 450°F. Roll out dough on a lightly floured surface to fit in a 9-inch round pie plate. Place crust into the dish, leaving edges rough. Place dried beans or pie weights into crust before baking for 8 minutes. Cool before adding filling. Reduce oven temperature to 425°F.
Blanche spinach in boiling water for 1-2 minutes, then transfer to a bowl filled with ice water to stop the cooking process. Transfer spinach to a towel to remove excess moisture.
Sauté onion with butter, ½ teaspoon garlic powder, ½ teaspoon salt, and ¼ teaspoon pepper over medium-high heat until onions are softened in texture and translucent in color. Turn off heat and set aside to cool.
Beat eggs with ½ teaspoon salt and ¼ teaspoon black pepper. Add half-and-half and ¾ cup cheese, whisk until well combined. Gently fold in spinach leaves and cooled sautéed onions then pour egg filling into pie crust. Sprinkle remaining cheese over top then bake at 425°F for 12-15 minutes. Lower temperature to 350°F and continue to bake for 22-25 minutes. Quiche is ready when a toothpick inserted in center comes out clean.
Cool for 20-30 minutes to allow quiche to set before slicing to serve.