In small saucepan, stir together sugar, mustard and salt, if desired, until blended. Stir in juice, vinegar and eggs until thoroughly blended. Cook over low heat, stirring constantly, until mixture is thick enough to coat a metal spoon with a thin film and a thermometer shows 160° F, about 6 minutes.
Remove from heat. Cool quickly by setting pan in bowl of ice or cold water and stirring for a few minutes. Cover. Refrigerate to chill thoroughly. Keep refrigerated when not being served.