01 - In a large bowl, whisk together flour and salt. Cut in cold butter until mixture resembles coarse crumbs. Gradually add ice water, one tablespoon at a time, mixing until dough just holds together. Shape into a disk, wrap with plastic wrap, and refrigerate for 30 minutes.
02 - Preheat oven to 375°F. Roll chilled dough on a lightly floured surface and place into a 9-inch tart pan. Trim edges and prick bottom with a fork. Line with parchment and fill with pie weights. Bake for 15 minutes, remove weights, then bake an additional 5 minutes until lightly golden. Let cool slightly.
03 - Heat olive oil in a skillet over medium heat. Cook onion until soft, about 3 minutes. Add mushrooms and cook until tender, approximately 5 minutes. Stir in spinach and cook until wilted, 1 to 2 minutes. Remove from heat and allow to cool slightly.
04 - Whisk eggs, heavy cream, salt, pepper, and nutmeg in a large bowl. Fold in sautéed vegetables and shredded cheese until evenly combined.
05 - Pour filling into crust. Bake for 30 to 35 minutes until center is set and top is lightly golden.
06 - Allow quiche to cool for at least 10 minutes before slicing and serving.