Heat 2 tablespoons of the oil in a large skillet over medium heat. Add the celery, carrots and onions, season with salt and pepper and cook until softened, about 10 minutes. Stir in the garlic and cook for 30 seconds. Add the tomato paste and cook for a minute. Pour in the red wine and cook until it is mostly evaporated, about 3 minutes. Pour this mixture into a slow cooker.
Pour the remaining 2 tablespoons oil into the skillet, add the beef and season with salt and pepper. Cook, breaking up any lumps, until it is nicely browned, about 15 minutes. Drain off any excess fat. Stir in the milk and cook until it is mostly absorbed by the meat, about 5 minutes. Add this to the slow cooker along with the tomatoes, oregano, basil, thyme, red pepper flakes, nutmeg and Parmesan rind if using. Add some salt. Give it a good stir, cover and cook on low for 6 hours.
Skim off any fat on top and check the consistency. If it is too thick, add a bit of water. If it is too thin, cook for another 30 minutes with the cover off.
Remove the rind and serve over spaghetti with lots of grated Parmesan and topped with fresh basil and parsley. Extra sauce can be frozen for later use.