Here’s what I found at foodreference.com, similar to what I read elsewhere. I copied and pasted:
DEEP DISH PIZZA
Italians started coming to Chicago from Italy during the 1850s, by the 1940’s there was a significant Italian population of Italian immigrants and their descendants. Many had been successful in the restaurant and bar businesses.
There are some stories about U.S. soldiers of Italian descent returning from Europe after World War II and experimenting with different pizza recipes, and eventually creating deep dish pizza. The only problem with these stories is that deep dish pizza was being sold in Chicago in the early 1940’s, before the end of the war.
The one story that is probably true is about a man named Sewell (maybe originally from Texas) who created the deep dish pizza in 1943 at his bar and grill, Pizzeria Uno. It was so popular that he soon opened another place called Pizzeria Due. Soon other restaurants were serving deep dish pizza, including several opened by former employees of Sewell. (A former employee of Pizzeria Uno, Rudy Malnati claims to have created the original deep dish pizza too) Deep dish pizza became popular with more and more people, and soon Chicago became known for creating it. Everyone, not just Italians adopted it as a ‘Chicago’ food.
Sorry, Michael, no mention of Betty Crocker. Also, I don’t think that Chicago Deep Dish Pizza is what most Americans think of as a casserole. I’m sorry you don’t care for it.