1Pick over and rinse the lentils, and put them in a medium saucepan. Put the thyme, bay leaves, garlic and peppercorns on a square of muslin or jelly bag. Tie into a little pouch with string, rubber band or clothes peg. Add the pouch to the pan along with the onion and carrot. Fill the pan with cold water to cover the lentils by about 2 inches and bring to a boil over medium-high heat. Immediately lower to a gentle simmer and simmer, uncovered, until just tender (30 to 40 minutes).
2While the lentils are simmering put the sausage in a small saucepan or deep skillet with enough water to cover by about 1cm. Bring to a bare simmer. Poach for about 10 minutes.
3While the lentils and sausage are cooking make the vinaigrette: In a medium bowl, mix the vinegar with the mustard and a pinch of salt. Mix in the olive and walnut (if using) oils. Season to taste with salt.
4Drain the sausage, cool under cold water and peel off the casing. When the lentils are cooked drain them and discard the herb bag and vegetables. Add the dressing to the lentils while they are warm.
5Slice the peeled sausage into 1/4-inch rounds and quickly griddle. Add the sausage to the lentils, tossing to coat. Stir in the parsley and spring onions/scallions, and season with a generous amount of black pepper, plus more salt and vinegar to taste.