Preheat oven to 375°F. Add olive oil and butter in a pan on medium heat. Add potatoes and cook for about 5 minutes. Then and shallots and cook until softened. Meanwhile, make the sauce by combining all the ingredients in a bowl. Season chicken with salt and pepper.
Transfer the shallot and potatoes and arrange the chicken in a baking dish. Pour the sauce over the top and sprinkle a few torn rosemary leaves. Loosely cover with foil and bake for 15 minutes. Remove foil and continue baking for about 20-25 minutes or until golden brown and fully cooked.