1. Preheat oven to 375ºF. In a small skillet over medium heat, melt butter. Add onion and sauté until translucent, about 5 minutes. Add garlic and cook for 1 minute longer, stirring. Transfer to a small bowl and let cool.
2. Sprinkle onion mixture, spinach and Gruyère over bottom of pie shell. Beat eggs and milk together, season with salt, pepper and nutmeg. Gently pour into crust.
3. Bake quiche for 40 to 45 minutes, until set and nicely browned. Remove from oven and let rest for 10 minutes.