Boil the pasta in plenty of water until cooked al dente. Drain, and set aside (don't worry if it sticks together a little as it cools - it will be fine once you stir it into the sauce!).
While the pasta is cooking, prepare the sauce. Melt 1tbsp butter in a large frying pan with a dash of oil, and add the finely chopped onion and garlic. Cook over a medium-low heat for 5-10 minutes, stirring regularly, until soft and translucent (you don't want it to brown).
Add the remaining butter to the pan with the onions, and once it has melted, add the flour. Mix well to form a roux - it should be a thick paste. Cook for a minute or so, stirring constantly, then add a dash of milk (half a cup or so). Mix well until all the milk has become incorporated, then add some more. Add the bay leaves, and continue to add the milk a little at a time until you end up with a smooth sauce (apart from the small lumps of onion!). Simmer the sauce for a few minutes until it has thickened slightly, and add the nutmeg and a generous amount of salt and pepper.
Add the cheese to the sauce (hold back a bit of the cheddar for topping), and mix to combine. Cook for a few more minutes until the cheese has completely melted. Find the bay leaves, and discard them.
Add the cooked pasta, and mix to combine. Transfer the mixture into a large baking dish (mine was a deep dish that measured around 11 x 9 inches), and sprinkle over the remaining cheddar and the breadcrumbs. Bake at 190°C (Gas Mark 5 / 375°F) for around 30 minutes, or until the topping has crisped up to your liking.
Garnish with a little dried parsley if desired!