- 3 medium navel oranges
- 1 large pink or red grapefruit
- 3 tablespoons sugar
- 1 large egg
- 3 tablespoons vin santo or other sweet dessert wine
- 3/4 teaspoon cream of tartar
- Garnish: freshly ground nutmeg
- 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.