upon request — when asked for; only if someone asks for it

a chef stands at the cooking station holding a bottle of special hot sauce while a waiter approaches with an empty hand gesture. The waiter points toward a dining table where a customer is raising their hand and making a specific asking motion. The chef nods and hands over the hot sauce only after seeing the customer's request. Other kitchen staff continue their work while the waiter walks toward the requesting customer with the sauce. the special sauce being provided only when asked for by the customer