Cut peel and white pith from oranges and grapefruit with a sharp knife, then cut sections free from membranes.
Drain fruit in a large sieve set over a bowl 5 minutes, reserving juice for another use. Toss fruit with 1 tablespoon sugar.
Beat egg, remaining 2 tablespoons sugar, vin santo, cream of tartar, and a pinch of salt in a double boiler or a metal bowl set over a pot of simmering water with a hand-held electric mixer at medium speed until slightly thickened and frothy, about 5 minutes (mixture will froth up and then subside a bit before thickening).
Remove from heat.
Divide fruit among 4 stemmed glasses and top with zabaglione.
Serve immediately.
The eggs will not be fully cooked in this dessert, which may be of concern if there is a problem with salmonella in your area.
