Combine all of the ingredients in a saucepan and bring to a boil while whisking constantly.
Serve immediately, or reserve in a thermal container during service. If the latter, shake well before serving. Depending on the quality of the container, it can remain hot from 2 to 8 hours. Make sure to check the internal temperature every hour. If it drops below food-safe holding temperatures, it needs to be re-heated above 62°C / 145°F to prevent bacteria growth. Discard any remaining liquid in the thermal container after each service.