Start the pasta water. Fill a large pot with water, add a generous pinch of salt, and bring it to a rolling boil. While it heats, you can begin the sauce. The gentle rumble of the boiling water is the background music for this dish.

Brown the mushrooms properly. In a large frying pan or heavy-based skillet, heat the olive oil over a medium-high heat until it shimmers. Add the sliced mushrooms in a single layer — you might need to do this in two batches. Leave them undisturbed for a full 3-4 minutes until the undersides are a deep golden brown. You should hear a steady, pleasing sizzle; if the pan goes silent, it's too crowded. Only then should you stir them. The aroma will shift from raw fungus to a rich, nutty scent. Once browned, transfer them to a plate.

Sweat the aromatics. Reduce the heat to medium. Add the butter to the same pan. Once it has melted and is foaming gently, add the diced onion. Cook for 5-6 minutes, stirring occasionally, until the onion is soft and translucent — it should look glossy and smell sweet, not burnt. Add the minced garlic and cook for one more minute until fragrant, which takes about the time it takes to count to sixty slowly.

Make the roux. Sprinkle the flour over the softened onions and garlic. Stir continuously with a wooden spoon for 2 minutes. The mixture will form a thick, sandy paste that clings to the spoon. The colour should be a pale biscuit — if it turns brown, your heat is too high. The smell will be of cooked dough, warm and toasty.

Build the sauce. Slowly pour in the room-temperature milk, a little at a time, whisking constantly. After each addition, whisk until the sauce is completely smooth before adding more. You'll feel the sauce thicken noticeably on the spoon. Once all the milk is in, bring the sauce to a gentle simmer — look for small bubbles breaking the surface, not a furious boil. Let it cook for 3-4 minutes, stirring often, until it coats the back of a spoon thickly enough to leave a clear trail when you drag your finger through it.

Finish the cheese sauce. Remove the pan from the heat. Stir in the grated nutmeg, Dijon mustard, and a good pinch of salt and pepper. Add the grated cheddar and mozzarella, one handful at a time, stirring until each batch is fully melted and the sauce is glossy and smooth. The colour will be a pale, creamy gold, and the texture should be like thick double cream.

Cook the pasta and wilt the spinach. By now, your pasta water should be boiling. Add the rotini and cook according to the package instructions until al dente — about 10-11 minutes. Just before you drain the pasta, ladle out a cup of the starchy pasta water and set it aside. Drain the pasta well. Return the sauce to a low heat and add the fresh spinach in handfuls, stirring until it wilts down into the sauce. You'll see it shrink from a big pile to a dark green ribbon in the golden sauce.

Combine and rest. Add the drained pasta and the reserved browned mushrooms to the sauce. Stir everything together over a low heat for one minute, letting the sauce coat every spiral. If the sauce seems too thick, loosen it with a splash of the reserved pasta water. Taste and adjust the seasoning with salt and pepper. Serve immediately in warm bowls, topped with a few fresh thyme leaves if you like. After testing this recipe five times, I finally got it just right — the key is that patience with the browning and the roux.