150 g (5 oz) medium or mature Cheddar cheese, grated
pinch of ground nutmeg (optional)
freshly ground black pepper
Cook the macaroni in a large saucepan of lightly salted boiling water for 8–10 minutes, or according to packet instructions, until al dente.
Meanwhile, heat the oil in a frying pan and cook the bacon for 3–4 minutes, until cooked and lightly golden. Add the mushrooms and cook for a further 3–4 minutes, until softened. Remove from the heat and set aside.
Pour the milk into a saucepan with the flour and butter, and cook over a medium heat, whisking constantly, until thickened and simmering gently. Cook for 2–3 minutes, then remove from the heat and stir in half the grated cheese.
Drain the pasta and combine with the cheese sauce, bacon and mushrooms. Add the nutmeg, if using, and season with black pepper. Tip into a large ovenproof dish, scatter over the remaining cheese and cook under a preheated grill for 5–6 minutes, until golden and bubbling. Serve immediately.