Prepare the mushrooms. Slice the mushrooms into even, chunky slices about ½ cm thick. Do not slice them paper-thin; they need substance to brown properly. Set them aside on a clean tea towel to absorb any excess moisture — dry mushrooms sear, wet mushrooms steam.

Bring a large pot of salted water to a rolling boil. Add the fettuccine and cook until al dente, about 1 minute less than the packet suggests. The pasta should offer gentle resistance when bitten. Reserve a mugful of the starchy pasta water before draining, then drain the pasta and toss it with a drizzle of olive oil to stop it sticking.

While the pasta cooks, heat a large, heavy-based frying pan over a medium-high heat. Add 1 tablespoon of the butter and the olive oil. When the butter has melted and starts to foam, add the mushrooms in a single, uncrowded layer. Let them cook undisturbed for 3–4 minutes — you will hear a lively sizzle and see the undersides turn a deep, golden brown. Flip and cook for another 2 minutes until the mushrooms are glossy, shrunken, and smell intensely nutty. Transfer them to a plate.

Lower the heat to medium. Add the remaining 3 tablespoons of butter to the same pan. Once it melts and begins to foam, add the diced onion. Stir and cook for 4–5 minutes until the onion turns translucent and soft, and you catch a sweet, mellow aroma. Add the sliced garlic and the fresh thyme leaves, stirring constantly for 30–40 seconds until the garlic is fragrant but not coloured — burnt garlic will turn bitter.

Pour in the double cream and the stock. Stir everything together, scraping the bottom of the pan with a wooden spoon to lift any browned bits. These caramelised remnants are packed with flavour. Bring the sauce to a gentle simmer — you should see lazy bubbles at the edges, not a violent boil. Let it cook for 3–4 minutes, stirring occasionally, until it thickens enough to coat the back of that spoon.

Season the sauce with salt, pepper, and the freshly grated nutmeg. Add the grated Parmesan in handfuls, stirring after each addition until it melts completely. The sauce will turn a pale ivory colour and smell rich and savoury. Return the browned mushrooms to the pan, along with any juices that have collected on the plate, and stir to combine.

Add the drained fettuccine directly to the sauce. Toss everything together with tongs for a full minute, letting the pasta drink up the cream. If the sauce seems too thick, splash in a little of the reserved pasta water — the starch will help it cling beautifully. The final texture should be silky and luxurious, each strand glistening.

Serve immediately in warm bowls. Garnish with a generous shower of chopped parsley, extra Parmesan, and a final crack of black pepper. Eat while the sauce is still glossy and hot.