Put the lentils in a pan of lightly salted, cold water and bring to the boil. Simmer for about 15 minutes until soft (but should not fall apart).
Chop the onion, cut the carrot into small cubes.
Cut off the fronds, set aside. Remove the outer, woody layer, then slice the rest into thin strips or small pieces.
Heat the oil in a pan. When hot, add the chopped onion, carrot, fennel and fennel seeds. Season with salt & pepper. Simmer under cover and over low heat until the veggies are soft.
Meanwhile, cook the sausages in the oven, or grill/fry them.
Drain the lentils, add to the vegetables, together with a little cooking water. Add the chopped parsley, vinegar, and chopped fennel fronds. Stir and taste, then adjust the seasoning, if needed.
Transfer the vegetable mixture to a large platter. Cut the sausages into thick slices and place them on top of the vegetables.
Serve warm.