Peel and finely chop the onion. Peel the skin of the sausage. Finely dice sausage and cucumber. Mix mustard, vinegar, salt, pepper and sugar. Whip the oil into the mixture. Mix onion, sausage and cucumber and pour the vinaigrette over it.
Allow a little to soak through. Wash the parsley, dab dry and chop finely. Put some crème fraîche on each baguette slice and spread the salad on top. Sprinkle with parsley. Serve 2 slices of baguette on a plate, garnished with salad, parsley and cherry tomatoes