The Big Mac uses the two of the same patty used in the regular burger and the cheeseburger, a 1.6 ounce slab of frozen minced beef with dehydrated onions added when placed on the grill for cooking.
Two all-beef patties, special sauce, lettuce, cheese, pickles, onions on a sesame seed bun.
You do have the option of requesting no onions.
The article says they are mixing the onions into the meat so you won’t be able to get a Big Mac without onions