Though little known on the North Side of Chicago, the sweet steak has been attracting long lines of fans to South Side eateries for 50 years now. The sandwich starts with a steamed bun, piled high with grilled onions, chopped ribeye steak, American cheese, sweet pepper relish, sliced tomatoes and hot peppers — all doused in a signature reddish sweet sauce. Curious City’s Monica Eng digs into the origins of the sandwich and what’s in store for the future. Hint: it has to do with walnuts.