Combine the first 10 ingredients in a large shallow dish or a heavy-duty plastic zip-lock freezer bag; add pork.
Cover or seal and refrigerate 8 hours, turning occasionally.
Note: you may need to divided the marinade between two zip-lock freezer bags and put one roast in each bag.
Remove pork from marinade (reserve marinade and add water, if necessary to equal 2 1/2 cups).
Place pork on a rack in a roasting pan lined with foil.
In a small saucepan, whisk reserved marinade and cornstarch; place pan over med-high heat.
Bring to a boil, whisking constantly.
Cook, whisking constantly, 2-3 minutes or until thickened.
Remove and reserve 1/4 sauce; set remaining sauce in saucepan aside.
Broil pork 6 inches from heat for 5 minutes.
Pour 1 1/2 cups water into pan.
Lower oven temperature to 325°; bake 1 hour and 30 minutes or until meat thermometer reads 155°, basting with reserved 1/4 cup sauce during the last 15 minutes.
Remove roast and let stand 10 minutes or until meat thermometer reads 160°.