Instant Pot Chicken Thighs Marbella

Name: Instant Pot Chicken Thighs Marbella
Ingredients: 2 tablespoons extra-virgin olive oil; 4 pounds bone-in, skin-on chicken thighs (about 10 to 12), patted dry; Kosher salt and freshly ground pepper; 1 pound pearl onions, peeled (thawed if frozen); 3 cloves garlic, smashed and peeled; 1 ¼ cups balsamic vinegar; 1 cup low-sodium chicken broth; 2 teaspoons chopped fresh thyme; 1 dried bay leaf; 6 ounces pitted prunes, halved (1 cup); 2 ounces pitted and halved Spanish green olives, such as Castelvetrano (½ cup); 2 tablespoons unsalted butter; Cooked polenta or mashed potatoes, for serving