A retro dish with timeless flavors, this winning chicken dinner offsets salty, briny olives with sweet, dark prunes. You can get it on the table in under an hour, thanks to the Instant Pot or pressure cooker.
Heat oil in a 6- to 8-quart multicooker set to the high "Saute" setting. Season chicken on both sides with salt and pepper. Working in batches, add chicken to pot and cook until golden brown, 5 to 6 minutes. Transfer to a plate. Remove all but 2 tablespoons fat from pot.
Add onions and garlic; season with salt and pepper. Cook, stirring occasionally, until lightly caramelized and slightly softened, 6 to 8 minutes. Add 1/2 cup vinegar and cook, scraping up any browned bit from bottom of pot, until vinegar evaporates, 1 to 2 minutes. Stir in remaining 3/4 cup vinegar, broth, thyme, bay leaf, prunes, and olives. Return chicken to pot. Secure lid of multicooker.
Cook on high "Pressure Cook" setting for 10 minutes. Once time is complete, allow pressure to release naturally. Carefully remove lid. Discard bay leaf. A thermometer inserted into thickest part of chicken (near but not touching bones) should read 165 degrees, and onions should be tender. Transfer chicken to a plate. Using a slotted spoon, remove onions, garlic, prunes, and olives from liquid; transfer to plate. Whisk butter into liquid in pot; adjust seasoning as desired. Serve chicken mixture over polenta or mashed potatoes, with sauce spooned over top.