Mix the flour with the butter cut into small pieces and rub it with your fingertips.
Add the water, egg yolk and salt.
Knead and leave the dough to rest for 30 minutes in a cool place.
Roll out the shortcrust pastry on a floured work surface and place it in a tart pan.
Pierce the pastry with a fork and set aside.
Slice the red onion into thin rings, slice the mushrooms and cut the butternut squash into balls with a melon baller.
Preheat the oven to 180°C.
Fry the vegetables in a drizzle of olive oil: the squash should be tender.
Add the spinach at the end of cooking and season.
Whisk the eggs with the cream and milk in parallel.
Spread a first layer of mozzarella on the bottom of the pie, add the vegetables, pour the quiche mixture and sprinkle with the remaining mozzarella.
Place a few walnuts on the quiche and bake for 45 minutes.