Heat the oven and prepare your pan. Preheat the oven to 190°C (170°C fan, gas mark 5). Use a 25 cm oven-safe non-stick or cast-iron skillet. If your pan handle is not ovenproof, wrap it tightly in foil — you will thank yourself later.

Whisk the eggs. In a large bowl, beat the eggs with the milk, half the grated cheese, the salt, pepper and nutmeg. Whisk for a full 30 seconds until the mixture is uniform and slightly frothy. You should see tiny bubbles on the surface — that aeration helps the frittata rise evenly.

Soften the onion. Warm the olive oil and butter in the skillet over medium heat. Add the onion and cook for 4–5 minutes, stirring occasionally, until it turns translucent and glossy. It should smell sweet rather than sharp, and you will hear a gentle sizzle rather than an aggressive spit.

Brown the mushrooms. Add the sliced mushrooms in a single layer and leave them undisturbed for 3 minutes. They will release their liquid, then it will evaporate and they will start to colour. Stir and cook another 4–5 minutes until they are deep golden brown at the edges and have shrunk noticeably. This browning is where the flavour lives, so do not rush it.

Add garlic and spinach. Stir in the garlic and cook for 30 seconds until fragrant. Add the spinach in handfuls, stirring as it wilts. Within about 90 seconds it will collapse down to a fraction of its volume and turn a vivid, glossy green. Spread everything evenly across the pan.

Pour in the eggs. Reduce the heat to low and pour the egg mixture over the vegetables. Do not stir — let it settle into an even layer. Scatter the feta and remaining grated cheese over the top. Cook undisturbed for 4–5 minutes, until the edges look set and slightly pulled away from the sides of the pan, while the centre still jiggles when you nudge the handle.

Bake to finish. Transfer the skillet to the oven and bake for 12–15 minutes. The frittata is ready when the centre is just set, the top is puffed and pale gold, and a knife inserted into the middle comes out clean. It will smell nutty and rich, with no raw egg aroma remaining.

Rest before slicing. Remove from the oven and leave it in the pan for 5 minutes. The residual heat finishes the cooking gently and the structure firms up. Run a silicone spatula around the edge, then slide it onto a board or serve straight from the pan. Scatter with chives and cut into wedges.