Prepare the Pastry: Preheat your oven to 200°C (180°C fan). Lightly dust a surface with flour and unroll your pastry sheet. Use it to line a 23cm loose-bottomed flan tin, gently pressing it into the sides. Trim any excess pastry. Prick the base all over with a fork.

Blind Bake the Crust: Line the pastry case with a sheet of baking parchment and fill it with baking beans or uncooked rice. Bake for 15 minutes. Carefully remove the paper and beans, then return the pastry case to the oven for another 5 minutes until it's pale golden. This process, known as blind baking, is crucial for a crisp base.

Cook the Vegetables: While the pastry bakes, heat the olive oil in a large frying pan over a medium heat. Add the chopped onion and cook for 5-7 minutes until soft and translucent. Add the crushed garlic and sliced mushrooms and cook for another 5-8 minutes until the mushrooms have released their liquid and are nicely browned.

Wilt the Spinach: Add the fresh spinach to the pan in batches. It will look like a lot, but it wilts down significantly. Cook for 2-3 minutes until completely wilted. Tip the entire mixture into a sieve and press firmly to remove as much liquid as possible. Set aside.

Make the Custard Filling: In a large mixing bowl or jug, whisk together the eggs, double cream, and milk. Season generously with salt, black pepper, and the ground nutmeg. Stir in about two-thirds of the grated cheddar cheese.

Assemble the Quiche: Reduce the oven temperature to 180°C (160°C fan). Scatter the cooked mushroom and spinach mixture evenly over the base of the pre-baked pastry case. Carefully pour the egg custard mixture over the top.

Bake to Perfection: Sprinkle the remaining grated cheese over the surface of the quiche. Bake for 20-25 minutes, or until the filling is golden brown and just set. What works best for me is checking for a slight wobble in the very centre – it shouldn't be liquid, but a gentle jiggle is fine as it will continue to set as it cools.

Rest and Serve: Remove the quiche from the oven and let it cool in the tin for at least 10 minutes before carefully removing it. This helps it to firm up and makes it much easier to slice. Serve warm or at room temperature.