For the sauce:
75ml double cream
Freshly ground nutmeg (optional)
175g cheddar cheese, grated
For the topping:
2 tbsp freshly grated Parmesan cheese
1 tbsp breadcrumbs (optional)
Cayenne pepper (optional)
Bring a large pan of salted water to the boil, add a few drops of olive oil and cook the penne for exactly 10 minutes Drain and rinse with cold water, and set aside.
In large, heavy frying pan melt 50g of butter then add the leeks and bacon. Cook, stirring occasionally, over low heat until the leeks are browning and the bacon is somewhat crispy.
Meanwhile, make a white sauce by putting the remaining the butter, the flour, and milk in a medium saucepan. Bring gently to a simmer and stir until thick and bubbly. Stir in the cheese and add a generous grinding of black pepper, and some nutmeg.
In a large bowl, mix together the pasta, leek mixture and cheese sauce. Place in one large gratin dish. Sprinkle with the Parmesan cheese and breadcrumbs, then dust lightly with cayenne pepper.
Bake at 180C for 30-40 minutes until nicely brown and bubbly. If prepared ahead and refrigerated, allow for a few minutes extra cooking time.