Step 1

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.

Step 2

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.

Step 3

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.