Wash the lamb's lettuce and shake dry.
Coarsely chop the walnuts and toast them in a dry pan until aromatic. Remove the nuts from the pan and allow to cool.
Cut the ham into strips. Juice the lemon.
Mix the lemon juice with the oil, mustard, salt and pepper, and beat with a whisk.
Clean and wash the fennel. Shake the leafy tops dry, then finely chop and add to the vinaigrette.
Wash, quarter and core the apple. Peel the onion. Mix the lettuce with about a third of the vinaigrette.
Cut or plane the fennel, apple and onion into very fine slices and mix with the remaining vinaigrette. Arrange on 2 plates, add the lettuce and top with ham and walnuts.