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Mississippi Salisbury steaks are an unique spin on the classic dinner. Pepperonicinis add tang and acidity to meaty steaks and a deeply savory gravy.
Combine ground beef, panko, ranch dressing mix, egg, and 1/2 cup of the onion in a large bowl. Using clean hands, mix well until all ingredients are just combined.
Shape beef mixture into 4 (5 1/2-oz. each) oblong patties (about 3/4-inch thick). Place on a parchment paper-lined rimmed baking sheet. Press with your thumb to make a shallow (1 1/2-inch) indentation in the center of the top side of each patty.
Heat oil in a large cast-iron skillet over medium until shimmering. Add patties, dimpled sides up; cook until patties are well browned on both sides and a meat thermometer inserted into center of patties registers 145°F, 4 to 5 minutes per side. Transfer to a platter. Do not wipe skillet clean.
Add flour and remaining 1/2 cup onion to drippings in skillet. Cook over medium, stirring constantly, until onion softens and flour is lightly toasted, about 2 minutes.
Gradually whisk in beef stock, and bring mixture to a boil over medium-high. Stir in au jus gravy mix. Cook, stirring occasionally, until sauce is reduced and thick enough to coat the back of a spoon, about 3 minutes.
Return patties to skillet along with any accumulated juices; stir in pepperoncini peppers. Spoon sauce all over patties, and simmer gently until warmed through, about 2 minutes. Remove from heat.
Garnish with parsley. Serve with egg noodles.