Place all the ingredients in a bowl and stir with a fork or small whisk until the dressing emulsifies.
The salad
Wash the spinach and spin dry well.
Peel the grapefruit and cut out the fillets.
Wash the plums and cut into bite-sized pieces.
Cut the onion into fine rings.
In a large bowl, marinate the vegetables and fruit with the dressing.
Divide the salad between serving plates or bowls and top each portion with a mozzarella ball. Then tear a slice of ham with your fingers and garnish each portion with it.