Melt the butter with the oil in a large, heavy-based saucepan over a low heat. Add the onion, celery, carrot and bay leaf and cook, stirring occasionally, for 10 minutes until softened but not coloured. Add the meat, season with salt and pepper and cook, stirring, over a medium heat until no longer pink.
Pour in the wine and bring to the boil. Gently simmer for 15 minutes until evaporated. Stir in the milk and nutmeg and simmer for a further 15 minutes until the milk has evaporated. Stir in the tomatoes and cook, uncovered, over a very low heat for 3–5 hours. The sauce is very thick, so when it begins to stick, add 100 ml (3½ fl oz) of the stock at a time, as needed.
Cook the pasta in a large saucepan of salted boiling water until al dente: according to the packet instructions for dried pasta or for 2 minutes if you are using fresh pasta. Drain thoroughly, reserving a ladleful of the cooking water.
Return to the pan and place over a low heat. Add the sauce and stir for 30 seconds, then pour in the reserved pasta cooking water and stir until the pasta is well coated and looks silky. Serve immediately with a scattering of grated Parmesan.