Start by preparing the crust. Blitz together the dry ingredients for a minute, then add the cold butter. Run the food processor until the butter resembles small peas. Then whisk together the egg and milk and drizzle it in the food processor until the dough comes together. Bring the dough into a ball and wrap it in plastic wrap. Let chill in the fridge for at least an hour.
Preheat your oven to 170°C.
Roll out the dough between two pieces of parchment paper and place it in a 23cm loose-based tart tin. Put it in the fridge while you prepare the filling.
Sweat the onions in a pan with a knob of butter and a pinch of salt until translucent. Remove from the pan. Fry the chanterelles with some butter in the same pan for about 5 minutes. Do not cook them all the way through. If excess liquid develops, drain it with a sieve.
Scatter half of the grated cheese over the tart tin. Arrange the cooked chanterelles over the base. Whisk together the eggs, almond milk, salt and pepper. Then add the remaining cheese and pour the mixture over the tart tin. Place in the oven and let cook for 35 minutes.
Let cool for a few minutes before removing the quiche from the tin.