Preheat the oven to 180°C fan bake.
Combine all the ingredients except the pork in a large bowl and leave for 15 minutes.
Add the pork and mix until fully combined. Form into about 24 large walnut-sized meatballs.
Heat a little oil in a large frying pan and quickly brown the meatballs. Set aside.
Heat the oil and butter in a large frying pan and cook the onion, leek, garlic, fennel seeds and chilli with a good pinch of salt for 10 minutes or until very tender. Set aside.
Melt the 50 grams butter in a large saucepan and whisk in the flour until smooth. Cook over a low heat for 1 minute, then gradually whisk in the combined milk and cream, whisking continuously until smooth.
Stir in the mustard and nutmeg and season well. Simmer for 5 minutes, stirring often.
Remove from the heat and stir in the three cheeses until melted and smooth. Stir in three-quarters of the leek mixture and set the rest aside.
Cook the pasta in plenty of salted boiling water for 2 minutes less than the package instructions, then drain well.
Combine the pasta with the sauce and tip into the baking dish. Nestle in the meatballs, then dot over the remaining leek mixture.
Scatter over the combined leftover three cheeses and bake for 25 minutes, until golden and bubbling around the edges.