Use a melon baller to cut out balls of melon.
Chop the cherry tomatoes in half and the cucumber into small squares. Prepare the avocado last, cutting it into small chunks. Combine all the ingredients with vinaigrette dressing. Garnish with sprigs of fresh watercress.
To make the dressing - mix together olive oil, balsamic vinegar, mustard, garlic (finely crushed), ground black pepper, and some salt.