Prepare the crustIn a bowl, mix flour and salt. Rub in the cold butter until the mixture resembles crumbs.Add the egg and cold water and quickly form a dough. Shape into a disk, wrap, and refrigerate for 30 minutes.Roll out the dough and line a quiche pan. Prick the base with a fork and pre-bake at 180°C / 350°F for 10–12 minutes.
Prepare the spinach mixtureHeat olive oil in a pan. Sauté the onion until soft, then add garlic and cook briefly.Add the spinach and cook just until wilted. Season lightly with salt and pepper. Remove from heat.
Assemble the fillingIn a bowl, combine the cooked chicken, chopped sun-dried tomatoes, feta cheese and the spinach mixture.Spread evenly over the pre-baked crust.
Make the custardWhisk the eggs with the cream. Add mazzarella cheese, a pinch of nutmeg or herbs, and black pepper.Pour the custard evenly over the filling.
BakeBake at 180°C / 350°F for 35–40 minutes, until set and lightly golden on top.
Rest before servingRemove the quiche from the oven and let it rest for at least 1 hour.This allows the custard to fully set and makes slicing much cleaner.