Bring a large pot of generously salted water to a rolling boil. Add the pasta and cook according to package directions until al dente. Just before draining, reserve 200ml of the starchy cooking water. Drain the pasta and set aside.

While the pasta cooks, heat the olive oil in a large, deep frying pan over medium-high heat. Add the sliced mushrooms in a single layer — do not overcrowd the pan (cook in two batches if needed). Let them cook undisturbed for 3–4 minutes until the undersides turn deep golden brown and you hear a steady sizzle. Toss and cook for another 2 minutes until all sides are caramelised and the mushrooms have released their moisture and reduced in volume.

Reduce the heat to medium. Add the diced onion and cook for 3 minutes, stirring occasionally, until it turns translucent and softens. The onions should look glossy and pale gold. Add the sliced garlic and dried thyme, stirring for 30 seconds until fragrant — the smell should be savoury and aromatic, not burnt.

Pour in the stock and bring to a gentle simmer, scraping the bottom of the pan with a wooden spoon to lift any browned bits (this adds huge flavour to the sauce). Let it bubble for 2 minutes until slightly reduced.

Lower the heat to medium-low and pour in the double cream. Stir well to combine, then add the grated Parmesan. Keep stirring until the cheese has melted completely and the sauce is smooth and pale ivory in colour. Simmer gently for 3–4 minutes, stirring occasionally, until the sauce thickens enough to coat the spoon.

Add the fresh spinach leaves to the pan in handfuls, stirring gently. Within 1–2 minutes the spinach will wilt dramatically — the leaves will turn a vivid, dark green and the sauce will take on a slightly flecked appearance. Season with salt and pepper to taste.

Add the drained pasta directly into the sauce, toss well using tongs to coat every strand. If the sauce seems too thick, add a splash of the reserved pasta water (start with 2 tablespoons and increase as needed) until the consistency is silky and flows easily around the pasta.

Finish with a squeeze of lemon juice if desired — this brightens the richness without making the dish taste sour. Serve immediately in warm bowls, topped with extra grated Parmesan and a crack of black pepper.