Make the pastry: Blitz the flour, salt, and cold butter in a food processor until it resembles coarse breadcrumbs. Add the yolk and 2 tablespoons of ice water; pulse until the dough just clumps together. If it feels dry, add the last tablespoon of water. Turn it out onto a lightly floured surface, bring it together into a disc (don't knead), wrap in cling film, and chill for at least 30 minutes. The dough should feel firm and cool to the touch, not sticky.

Blind bake the case: Preheat your oven to 200°C (180°C fan). Roll the chilled pastry out on a floured surface to a thickness of about 3mm, until it's large enough to line a 23cm loose-bottomed tart tin. Gently press it into the tin, letting the excess hang over the edge. Prick the base all over with a fork. Line with baking paper and fill with baking beans or dried rice. Bake for 15 minutes, until the edges look dry and are just beginning to turn a pale gold. Remove the paper and beans, then bake for another 5-7 minutes until the base is a light, even golden brown and feels sandy to the touch. Set aside to cool slightly. I used to struggle with a soggy bottom until I discovered this thorough blind-baking technique—it's non-negotiable.

Prepare the filling: While the pastry cools, whisk the eggs in a large bowl until they are broken up and slightly frothy. Then, whisk in the double cream, milk, Dijon mustard, and a generous pinch of salt, pepper, and nutmeg. The mixture should smell rich and creamy, with a hint of spice from the mustard.

Layer the fillings: Sprinkle half the grated cheese over the base of the pre-baked pastry case. Evenly scatter the shredded chicken, sautéed mushrooms, and spring onions over the cheese. This layering ensures every bite has a bit of everything.

Pour and top: Gently pour the egg and cream mixture over the fillings, filling the case almost to the brim. Sprinkle the remaining cheese on top. The liquid should just cover the solid ingredients; if it looks too full, don't panic—it will settle as it bakes.

Bake to perfection: Reduce the oven temperature to 160°C (140°C fan). Carefully place the quiche on a baking sheet and bake for 40-50 minutes. The quiche is done when the centre is just set with a slight wobble—like a firm jelly. The top should be a beautiful golden brown, and the edges will be puffed up. A knife inserted into the centre should come out clean but moist. The smell of toasted cheese and butter will fill your kitchen.

Rest and serve: This is the most crucial step for a clean slice. Let the quiche rest in the tin on a wire rack for at least 20 minutes. This allows the custard to finish setting and firm up. Slice carefully with a sharp knife, wiping the blade clean between cuts.