Preheat the oven to **180°C (160°C fan)**. As it warms, take a moment to prepare everything with care. The spinach, once thawed, should be pressed completely dry in a clean tea towel or cheesecloth—this small detail ensures a quiche that sets beautifully. Finely mince the garlic, and slice the mushrooms into neat, even pieces, ready for the pan.

Warm half the oil in a skillet over a medium–high heat. Add the mushrooms and allow them to cook until they release their moisture and it evaporates away entirely, leaving them tender and lightly golden. This deepens their flavour and keeps the finished quiche from becoming waterlogged. In the final minute, stir through the garlic and a pinch of salt, just until fragrant, then set the mixture aside.

Lightly oil a 23cm pie dish, brushing the base and sides. Layer with intention: first the mushrooms, spread evenly; then the spinach, gently distributed; and finally the feta, crumbled over the top. Each layer brings its own texture and balance to the whole.

In a bowl, whisk together the eggs, milk, Parmesan, and black pepper until smooth and cohesive. Room-temperature eggs will blend more readily, giving a silkier custard. Keep the whisking gentle—just enough to bring everything together.

Pour the custard slowly over the layered filling, allowing it to settle into every corner. Finish with the mozzarella scattered evenly across the surface. Bake on the centre rack for around **50 minutes**, until the quiche is set at the edges with a soft, delicate wobble at the centre. It will continue to firm as it rests.

Remove from the oven and leave to stand for **5–10 minutes** before slicing. This brief pause allows the custard to settle fully, giving you clean, composed slices. Serve warm, or at room temperature—both are equally inviting.