Chicago deep-dish pizza, or just "deep dish," as it's commonly known, has befuddled pizza purists for decades.

Is it a pizza? Is it, as comedian Jon Stewart once quipped, "an [expletive] casserole?" Maybe it is more of a pie than a pizza pie? Or is it something else entirely? 

Regardless of what it is, deep dish has been an institution in Chicago since its creation in the 1940s, and the crusty, cheesy and sometimes meaty concoction is served to millions throughout the city each year.

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"Pizza tradition runs deep in Chicago, and Lou Malnati's deep-dish pizza is an original," Marc Malnati, owner of Chicago-style pizza chain Lou Malnati's, told Fox News Digital in an email.

Lou Malnati's is one of a handful of establishments that lays claim to the invention of the deep-dish pizza. It is believed to have been created in 1943 at Pizzeria Uno in Chicago, where Lou Malnati worked as a pizza chef, the Chicago Tribune reported. 

"Known for its buttery crust that is pulled up on the side of the pan to hold its fresh ingredients, it is hearty enough to satisfy Chicagoans as well as make it crave-worthy for all who try it," Marc Malnati said. "For over 53 years, Lou Malnati's has been serving up this deep-dish delight."

Unlike the more standard thin-crust pizza pie found on the East Coast, deep-dish pizza flips that concept on its head.

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"The order the ingredients are placed on a Chicago-style pizza is 'backwards' from a thin-crust pizza," the website for Lou Malnati's said.

After the crust is arranged along the sides of a deep baking dish, mozzarella cheese slices are then placed on the dough, the website said.

Then, any extras, such as meats or vegetables, are put on top of the cheese, it said. 

Sauce then goes on top, according to the website. 

Lou Malnati's uses "a fresh tomato sauce made with whole chunks of pear and plum tomatoes."

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Then, the whole thing is "topped off with a sprinkling of cheese and spices." 

Since the initial development of the deep dish, the item has been expanded (pun intended) by other innovative chefs in the Chicago area. 

"Although the origins of the Chicago-style deep-dish pizza go back to the early 1940s, Giordano's took that concept to the next level by focusing on deep dish 'stuffed' pizza since 1974," Yorgo Koutsogiorgas, president and CEO of Giordano's, told Fox News Digital in an email. 

The "stuffed" variety served by Giordano's features another layer of dough and is an "even thicker" version of the already hefty original deep dish, its website said.

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"To keep the thick concoction from dripping out into the pizza oven, and to help it cook effectively, a layer of dough was added over the top of the pie, with a hole in the center to vent steam," according to Giordano's website. 

 Traditionally, another layer of sauce is added on top of the second layer of dough, it said.

While a deep-dish pizza contains ingredients and structure similar to its thin-crust counterparts, there is one additional huge difference. 

That's how it is eaten.

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Using a fork and knife on a piece of thin-crust pizza may be viewed as unusual by many, but that is simply not the case with deep dishes.

"It’s completely appropriate here to slice off a chunk with your knife and lift it up with your fork," according to the website for Gino's East, another Chicago-based pizza chain.