Place pastry ingredients together in a food processor. Using the pulse switch, process the ingredients in a pulse motion until a smooth ball forms. Do not over mix. Transfer pastry to a floured surface and lightly knead to bring together. Roll the pastry into a circle slightly larger than the tin to fit either a 22cm flan ring or 28cm fluted quiche tin. Trim pastry and dock the base with a fork. Cover pastry with foil pressing it over the base, into the sides and over the edge of the tin. Fill with pastry weights or rice. Place in the freezer for 30-40 minutes until the pastry is cold and well rested.
Meanwhile, prepare the onions; heat butter in a frying pan and sauté onions over medium heat for 10 minutes until soft and translucent. Remove from heat and set aside. Combine eggs, cream, cheeses and season with freshly ground black pepper and salt, to taste.
Blind bake the pastry case in a preheated oven of 200°C for 12 minutes. Remove pastry weights and foil and cook for a further 5 minutes until pastry is dry and lightly golden. Remove from oven.
Cut four small vents in the pastry lid with a sharp knife. Brush with beaten egg mix and bake 35–45 minutes until the eggs are cooked through and the crust is golden brown and crispy.