Place flour with butter and salt in the processor and pulse until mixture resembles a coarse meal. Add water and process until dough comes together and is soft. Transfer dough to a lightly floured surface, form a ball and wrap in plastic wrap. Refrigerate for 1 hour.
Roll pastry between 2 sheets of plastic wrap or parchment paper to fit a loose bottom tart pan or a pie pan (8 ½ in / 21 cm). Trim dough and refrigerate for 30 minutes. Cover dough with parchment paper, put weights, and bake in a preheated oven to 350ºF (180ºC) for 15 minutes. Remove parchment paper and weights and continue baking 15 minutes more. Cool.
Melt butter in a skillet and add garlic and potato slices. Stir to coat potatoes with butter, season with salt and cover skillet. Cook for 5 minutes over low heat. Remove potato slices from the skillet with a slotted spoon and drain them on paper towel. Set aside.
Add leek to the same skillet, season and cook until tender. Remove from the heat and transfer leek to a bowl.
Place spinach leaves in a large saucepan with 3 tablespoons water. Cover saucepan and cook for 2 – 3 minutes or until leaves are wilted. Season with salt. Drain on a colander and cool. Dry spinach on paper towel or dish towel.
Spread half of the cheese on the cooked crust and arrange half of the cooked potato slices. Cover with half of the spinach and the leeks. Repeat layers.
Combine eggs with heavy cream and milk, season with salt, pepper and nutmeg. Pour over quiche and bake in the oven for 1 hour 15 minutes or until quiche is set.
Serve hot or at room temperature.