You might think you know all there is to know about McDonald’s, but a former employee has lifted the lid on what goes on behind the scenes.

Some former employees have divulged the mystery ingredients used for Big Mac burger sauce while others have shared how to get piping hot food.

Customers have even unveiled hidden truths about the fast food giant, like secret menus which have sparked fury among McDonald’s fans.

One ex-staffer from a Glasgow branch has revealed a different kind of secret. They claimed that staff sometimes take liberties with ingredients when supplies run low.

The insider claimed: “The majority of the time if we ran out of stock in specific types of onions or lettuce, we would just compromise and hope the customer didn’t notice.”

Speaking previously to The Sun, the former fast-food chain employee claimed that the substitutions mostly happen with salad items.

They elaborated: “For example, using dehydrated onions instead of fresh ones and shredded lettuce instead of iceberg.”

The source also pointed out that this practice isn’t confined to the golden arches, asserting: “This isn’t really a big secret as it happens in almost every industry in the country.”

According to the latest McDonald’s menu, only a few items still use iceberg lettuce. This includes the McCrispy, Vegetable Deluxe, McPlant, Bacon Mayo Chicken, McChicken Sandwich, and the McSpicy.

McDonald’s wraps and salads don’t use crunchy iceberg lettuce, instead opting for romaine, baby spinach and baby kale.

As for the onions, dehydrated onions are standard in most sandwiches, like the Big Mac and classic hamburgers. However, the Quarter Pounder uses fresh diced ones.

“McDonald’s uses both dehydrated onions (that have been rehydrated) as well as actual onions”, said Angel Livingood, a former manager at an Iowa McDonald’s.

A good way to tell if you’re eating dehydrated or actual onions is to look at how they’re cut. The small chopped onions are rehydrated while the slivered onion slices are fresh.

However, Angel claimed that you can ask for either on any sandwich. So if you prefer the fresh onions to the rehydrated ones, you can ask for those on a hamburger instead.

Express.co.uk has contacted McDonald’s for comment