Mix first six ingredients. Cut roast in half; sprinkle with salt and pepper. In a large skillet, heat oil over medium heat; brown roast on all sides., Transfer to a 5-qt. slow cooker. Pour pineapple mixture over top. Cook, covered, until a thermometer inserted in roast reads at least 145°, 3-4 hours., Remove roast from slow cooker; keep warm. Transfer cooking juices to a saucepan; bring to a boil. Mix cornstarch and water until smooth; stir into cooking juices. Return to a boil; cook and stir until thickened, 1-2 minutes. Serve with roast.