The Shack Stack is described on the menu as a cheeseburger and a 'Shroom Burger topped with lettuce, tomato, and ShackSauce.
The UK version cost £8.95, or around $12.21, and weighed 284 grams (10 ounces). It contained a beef burger slathered in melted cheese, a fried portobello mushroom stuffed with more cheese, two slices of tomato, and lettuce.
In the US, it cost $10.99, or around £8.03, and weighed 9.75 ounces (275 grams). It contained the same elements as the one in the UK, but the tomatoes and lettuce were smaller in New York.
Similar to the Smoke Shack, the ShackSauce was placed sparingly and was only visible on the top bun and lettuce. Mikhaila said more sauce would have made all the difference here, as this burger didn't seem to have as much flavor as the other burgers she tried.
Rachel agreed, and added that the beef patty didn't add much of anything except more weight.