Preheat the oven to 190'C.
Make sure the butter and water you are using to make the pastry are cold.
Place the flour, butter and salt in a bowl and rub between your fingertips until the mixture resembles fine breadcrumbs. Make a well in the centre and slowly add the water. Bring the pastry together by hand, kneading lightly until you have a smooth ball
Wrap the pastry in cling film and chill for at least 20 minutes.
Brush a deep, loose-bottom, 24cm tart tin with oil. Set a small ball of pastry aside and roll out the rest on a lightly floured surface. Lift the pastry into the tin using the rolling pin and press into the sides. Press any cracks together and let the excess overhang.
Use the small ball of pastry to press the tart case into the flutes around the sides of the tart case and trim the edges, saving a little more pastry for later. Prick the base lightly with a fork and chill the tart case, preferably in the freezer, for at least 30 minutes.
Line the pastry with baking parchment and fill with baking beans. Transfer to the oven and bake for 20 minutes. Remove the beans and the baking parchments and then return the pastry to the oven for 5 more minutes.
Reduce the oven temperature to 140'C.
Beat together the whole eggs, the yolks, crème fraîche, milk and chopped tarragon. Season with salt and pepper.
Peel and finely slice the onions. Thinly slice the mushrooms.
Sauté the bacon in a tablespoon of oil until browned and beginning to crisp. Add two tablespoons of oil and the onions to the pan. Cook for a further 5 minutes and then tip into a sieve to drain the excess liquid.
Add two more tablespoons of oil to the pan, add the mushrooms and cook over a high heat for 3 to 5 minutes. Add to the bacon and onions and then wilt the rocket in the pan.
Mix the cheeses and the wilted rocket into the custard and whizz with a hand blender. Once well combined, then stir in the rest of the filling ingredients.
Ladle as much filling into the case as you can - try to ladle the solid filling in first, then top up with custard. Place in the oven and bake for 40-50 mins until the quiche is set in the middle, turning occasionally to ensure the quiche cooks evenly.
Once cooked, trim the pastry edge using a small, serrated knife. Allow the quiche to cool for at least an hour before removing from the tin and serving in slices.
«The filling should be firm to the touch with retain a little wobble in the centre when cooked.»