Heat a dry frying pan over a medium-high heat and toast the walnuts for 3-4 mins until lightly golden and aromatic. Tip into a bowl and leave to cool.

Toss the apples with a splash of the lemon juice to prevent them discolouring. Combine the mayonnaise, yogurt, mustard and remaining lemon juice in a large bowl and season well. The dressing will keep chilled for up to two days. Mix the apples, grapes and celery with the dressing and toss well in a large bowl to ensure everything is evenly coated. Arrange the lettuce leaves on a serving platter and spoon over the apple mixture and any remaining dressing from the bowl. Scatter over the toasted walnuts before serving.