The McDonald's burger that kept its hot patty separate from cold toppings was the McDLT.
Introduced in the mid-1980s, the McDLT was designed around freshness: the hot beef patty and bottom bun occupied one side of a special two-compartment container, while the top bun, lettuce, tomato, cheese, pickles, and sauces sat on the other. Customers combined the two halves just before eating.
Its advertising famously urged customers to “keep the hot side hot and the cool side cool.” The name was short for “McDonald's Lettuce and Tomato,” highlighting the ingredients the packaging was meant to protect from heat and sogginess.
The McDLT was discontinued in the United States around 1991. Its bulky polystyrene container created environmental criticism and made the item awkward for restaurant kitchens. It is sometimes confused with the Big Mac or McDouble, but those burgers did not use separate hot-and-cold packaging.