Put a saucepan over a medium heat, add the oil and brown the chopped onion and garlic. Add the mushrooms and the peeled sliced potatoes. Add the water and the broth and boil it until the potatoes are cooked. Add the cream and boil for a few minutes. Whisk everything and leave it to one side.
Put the cream and fontina in a whipping siphon and keep it in the fridge. Crisp up the ham in a pan, microwave or oven. Make wafers with the parmesan, spreading it on a sheet of baking paper in the microwave and cook it for about 45 seconds.
Put the mushroom cream you have made (cappuccino) in a cup and finish with the cheese foam (mousse) and garnish with the ham and parmesan wafer.