Preheat the oven to 190°C (375°F), or 170°C (340°F) for a fan oven.

Place the pastry into a 23 cm (9-inch) tart or quiche tin. Press it gently into the edges, trim away any excess pastry, and prick the base several times with a fork.

Line the pastry with baking paper and fill with baking beans or dried beans. Blind bake for 10 minutes.

Remove the baking paper and baking weights, then return the pastry to the oven for another 5 minutes until the base is lightly golden. Set aside.

Heat the olive oil and butter in a large frying pan over medium heat. Add the chopped onion and cook for 3–4 minutes until softened.

Add the sliced mushrooms and cook for 6–8 minutes, stirring occasionally, until the mushrooms are golden and most of their moisture has evaporated.

Stir in the garlic and thyme and cook for another minute. Remove from the heat and allow the mushroom mixture to cool slightly.

In a large bowl, whisk together the eggs, cream, milk, salt, pepper, and nutmeg until well combined.

Spread the mushroom mixture evenly over the pastry crust, then sprinkle over most of the grated Gruyère cheese.

Pour the egg and cream mixture over the mushrooms and cheese. Sprinkle the remaining Gruyère on top.

Carefully transfer the quiche to the oven and bake for 30–35 minutes, or until the filling is set and the top is golden brown.

Allow the quiche to cool for 10–15 minutes before slicing. Garnish with fresh parsley and serve warm or at room temperature.