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Preheat the oven to 500F.
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In a small saucepan, whisk together the ketchup, brown sugar, beer, apple cider vinegar, Worcestershire sauce, fresh lemon juice, chili powder and mustard.
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Bring the pan to a boil then lower the heat and simmer the sauce for about 15 minutes, stirring occasionally.
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Season the chicken thighs with the kosher salt and black pepper. Put them in a 9 x 13 baking dish and pour the prepared sauce over them.
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Make sure the thighs are evenly coated with the mixture.
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Seal the dish tightly with aluminum foil and bake for 15 minutes in the preheated oven, then reduce the temperature to 300F and bake for 1 more hour.
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Remove the foil cover during the last 15 minutes of cooking to thicken the sauce and lightly brown the tops of the thighs.