Clean, wash and drain field and oak leaf lettuce. Pluck the oak leaf lettuce into bite-sized pieces. Peel and finely chop the onion. Peel 1 lime so that the white skin is completely removed. Remove the fillets from the parting skins with a sharp knife. Squeeze the rest of the lime.
Season juice with salt, pepper and sugar. Add onion and stir in the oil bit by bit. Coarsely chop the hazelnuts. Mix the salads with the vinaigrette, the lime fillets and the hazelnuts. Then arrange on plates