Clean the lettuce hearts, divide into leaves, wash and spin dry. If necessary, tear the leaves into smaller pieces and place in a bowl.
Wash the tomatoes, quarter and remove the stem and seeds.
Cut the tomato flesh into small cubes.
Mix the tomato cubes with the salad.
Wash the mussels, pat dry and fry in a pan in hot clarified butter until lightly browned.
Drizzle with 1 tbsp. dressing.
For the dressing, whisk mustard, lemon juice, salt and sugar. Add the oil and whisk.
Season the salad with the rest of the dressing and spread it on plates. Arrange the mussels next to and on top, lightly sprinkled with salt and serve immediately.