Squeeze the sausage meat out of the casings.
Heat a large Dutch oven over medium-high heat. (If your sausages are quite lean, add 1 tablespoon of olive oil.) Add the sausage meat and cook for 8-10 minutes, breaking it into large chunks and allowing it to brown well before stirring. Transfer to a plate, leaving the fat in the pot.
Reduce the heat to medium-low and add the onion, carrot and celery with a pinch of salt.
Cook for at least 15 minutes, stirring regularly, until very soft.
Stir in the garlic and fennel seeds if using and cook for a further minute.
Push the vegetables to the sides so there is a gap in the middle and add the tomato paste, letting it cook for about 3 minutes, stirring it regularly. This really helps to develop flavor.
Stir the tomato paste through the vegetables, then return the sausage meat to the pot, breaking up any large chunks with your spoon.
Pour in the white wine and let it reduce by about half, scraping up any browned bits from the bottom of the pot.
Add the milk, stock and tomatoes, mix well and bring to a very gentle simmer.
Add the bay leaves, lemon peel and the tied herbs.
Simmer uncovered on a very low heat for 60-75 minutes, it should just be at a very gentle simmer. Give it a gentle stir occasionally. If a layer of excess fat collects on the surface during cooking, feel free to skim some off the top.
The finished ragu should still have a little liquid remaining as it will continue to thicken when tossed with the pasta. Watch the video below for the finished consistency.
Remove the bay leaves, lemon peel and herbs and discard.
Taste and adjust seasoning, I usually add ¼ - ½ teaspoon pepper and a good pinch of salt. The amount of salt needed will depend on how well seasoned your sausages are.