Bring milk and cream to a simmer with citrus zests in a 2-quart heavy saucepan over medium-low heat.
Whisk together eggs, sugar, and salt in a medium bowl, then whisk in hot cream mixture.
Return mixture to pan and cook over medium-low heat, stirring constantly with a wooden spoon, until custard is thick enough to coat back of spoon (160° to 165°F on an instant read thermometer), about 2 minutes.
Immediately remove pan from heat and strain custard through a fine sieve into a bowl, discarding zests.
Serve warm, or at room temperature, or chill to thicken.
•Custard keeps, chilled, 3 days.•If your sauce creeps up above 165°F and looks like it's beginning to curdle, quickly remove from heat and whisk in 2 tablespoons cold milk, then strain.
