Waldorf Salad

Ingredients:

70g (2/3 cup) walnut halves
80ml (1/3 cup) olive oil
2 tbs white wine vinegar
1 tsp Dijon mustard
85g (1/3 cup) whole egg mayonnaise
1 tbs fresh lemon juice
2 red apples, quartered, cored, cut into thin wedges
1 celery stick, ends trimmed, thinly sliced
8 baby cos lettuces leaves
salt & pepper to season