Produced in a facility that processes milk, eggs, fish, shellfish, tree nuts, peanuts, wheat, and soybean.
Adjust rack to top position and preheat oven to 450 degrees. Wash and dry all produce. Halve buns. Halve, peel, and thinly slice onion. Cut potatoes into ½-inch-thick wedges (no need to peel).
Heat a drizzle of oil in a large pan over medium heat. Add onion and cook, stirring occasionally, until caramelized, 8-10 minutes. Stir in vinegar and 1 tsp sugar. Continue cooking until syrupy, 1-2 minutes more. Season with salt and pepper. Turn off heat; transfer to a small bowl. Wash out pan.
Meanwhile, toss potatoes on a baking sheet with a large drizzle of oil, half the Fry Seasoning, salt, and pepper. Roast on top rack, flipping halfway through, until browned and tender, 20-25 minutes.
Pat chicken dry with paper towels; season all over with salt and pepper. Heat a large drizzle of oil in pan used to caramelize onion. Add chicken and cook until browned and cooked through, 4-6 minutes per side. Evenly sprinkle chicken with cheese. Cover pan until cheese has melted, about 1 minute more. Turn off heat; remove from pan and set aside. Wipe out pan.
Melt 1 TBSP butter in same pan over medium heat. Add buns cut sides down and toast until golden, 1-2 minutes. In a second small bowl, combine sour cream and remaining Fry Seasoning; season with salt and pepper.
Spread as much sauce as you like onto bun bottoms, then top with chicken and caramelized onion. Divide sandwiches and potatoes between plates. Serve with any remaining sauce on the side for dipping.
Percentages are based on a diet of 2000 calories a day.