In a small pot over medium heat melt 1 tbsp butter. Mix in 1 tbsp flour to create a roux, then gradually add in the milk, whisking as you go to ensure no lumps form. Stir in parmesan, nutmeg and salt & pepper (to taste). Gently simmer for a few mins until it thickens. You can make the béchamel at the start and reheat it over low heat with a splash of milk to loosen it up if you need to (give it a good whisk to smooth out any lumps). Or, make it simultaneously as the pasta is cooking and the sauce is simmering (step 4).
Add a drizzle of oil to a large pan over medium heat. Add in the shallots & garlic and fry for a couple of mins to soften, then add in the sausage. Break it up with your wooden spoon and fry until golden and starting to form a crust. Stir in the crushed fennel seeds, then add the tomato puree and fry for a couple of mins.
Pour in the wine and simmer until the tomato puree re-thickens and the pungent smell of alcohol burns off. Pour in chicken stock and milk, then stir in oregano and chilli flakes. Gently simmer for 10 or so mins until nice and thick.
Meanwhile, add the pasta to salted boiling water and cook until al dente. Don't drain the water.
Use a large slotted spoon to transfer the pasta straight from the pot into the pan, embracing any water that comes with it. Stir through the sauce, then stir in parmesan. At this point you can stir through more of the leftover pasta water to loosen up the should if you need to.
Check for seasoning then serve up with a few spoonfuls of béchamel and a sprinkling of fresh basil leaves.