Remove from oven and carefully lift roast out of pan onto a plate. Cover with foil and allow to rest while completing sauce. Remove bay leaf and thyme sprigs and discard. Skim any excess fat from liquid and simmer over high heat to reduce liquid to about 2 cups, approximately 15 minutes. Whisk together heavy cream and cornstarch. Whisk into sauce along with the chopped thyme and bring back to a simmer. If sauce is not thick enough, continue to reduce for 2-3 minutes until a desired consistency is reached. The sauce will thicken slightly as it cools so be careful not to reduce too much. If you do, simply add a splash of heavy cream to adjust. Adjust final sauce with salt and pepper to taste.