01 - Set oven to 375°F.
02 - Mix flour and salt. Cut in cold butter until coarse crumbs form. Add ice water one tablespoon at a time until dough comes together. Shape into disk, wrap, chill 15 minutes.
03 - Roll dough on floured surface to fit 9-inch tart pan. Press into pan, trim excess. Prick base with fork. Line with parchment and fill with pie weights. Bake 10 minutes. Remove weights; bake 5 more minutes until lightly golden.
04 - Heat olive oil over medium heat. Cook onion until translucent, about 3 minutes. Add spinach, cook until wilted, about 2 minutes. Remove from heat and let cool slightly.
05 - Whisk together eggs, milk, cream, nutmeg, salt, and pepper until combined.
06 - Spread sautéed spinach and onion evenly over crust. Sprinkle with Gruyère cheese. Pour custard mixture over filling.
07 - Bake for 25 to 30 minutes until center is set and top is lightly golden.
08 - Allow to cool for 10 minutes before slicing and serving.