Peel and slice the cucumber and halve the tomatoes. Mix the salad leaves with the cucumber and tomatoes in a large salad bowl. Stone and peel the avocado, cut the flesh into dice and add to the bowl with the olives.
Make the dressing by whisking together the mustard, vinegar and oil. Season to taste with salt and pepper.
Pour the dressing over the salad, toss carefully to combine and serve.