Combine 1/3 cup sugar, flour, and salt in top of a double boiler; stir to remove lumps. Add eggs, stirring well.
Scald 1 1/2 cups whipping cream and milk; refrigerate remaining whipping cream. Gradually add hot cream mixture to egg mixture, stirring constantly. Cook over boiling water, stirring constantly, 10 minutes or until thickened. Cool. Stir in sherry. Spoon mixture into tart shells. Chill.
Beat reserved whipping cream until foamy. Add sugar; beat until stiff peaks form. Top each tart with whipped cream; sprinkle with nutmeg, if desired.