1 1/4 cups whole-wheat pastry flour (or half whole-wheat and half all-purpose); more for rolling
1/4 tsp. sea salt
1/8 tsp. aluminum-free baking powder
1/2 cup unsalted butter, chilled and cut into small pieces
1/4 cup ice water
3 cups 1/2-inch cubes peeled butternut squash
3 Tbsp. olive oil
1/2 tsp. grated nutmeg
2 cloves garlic, chopped
1 medium onion, peeled and thinly sliced
5 oz. baby spinach, about 6-8 cups
2 Tbsp. balsamic vinegar
3 large eggs, lightly beaten
1 cup grated Gruyère cheese
Freshly ground black pepper
Make the crust:
In a large bowl, combine flour, salt, and baking powder and mix well.
Add butter and cut into flour with your fingertips or two knives until the mixture is crumbly.
Gradually add water and mix until the mixture comes together.
Gather the dough into a ball. Flatten into a disk, wrap in plastic, and refrigerate for at least 1 hour.
Preheat oven to 400° F.
On a lightly floured surface, roll out dough to fit 10-inch fluted tart pan with removable bottom. Prick bottom of crust several times with a fork. Put a piece of parchment on top and fill with pie weights or dried beans.
Bake crust for 15 minutes. Remove weights and bake an additional 10 minutes, until edges of tart are brown.
Make the filling:
Preheat oven to 400° F. On a large baking sheet, toss butternut squash with 2 Tbsp. of the olive oil, 1/2 tsp. sea salt, and the nutmeg. Roast 20 to 25 minutes.
Heat remaining 1 Tbsp. of the olive oil in a large skillet over medium heat and sauté the garlic until it browns lightly, about 1 minute. Add onion and sauté until soft, about 3 minutes. Add spinach and a pinch of salt and sauté until wilted. Add balsamic vinegar and transfer to a large bowl.
Stir eggs into onion and spinach mixture, then fold in butternut squash and ¾ cup of the cheese. Season with sea salt and freshly ground black pepper. Mix well.
Pour mixture into tart pan. Sprinkle remaining 1/4 cup cheese on top.