For the bitter lettuce in grapefruit marinade, clean all the lettuce, wash and spin dry. Wash the radicchio first in lukewarm, then in cold water so that it loses bitterness.
Squeeze one grapefruit, peel and fillet the other. For the dressing, mix the grapefruit juice with mustard and vinegar, slowly stir in the walnut oil and season with salt, pepper and a pinch of sugar. Tear the bitter lettuce into bite-sized pieces, marinate with the dressing and drape the grapefruit fillets decoratively on top.