Combine chopped parsley and next 3 ingredients.
Butterfly pork roast by making a lengthwise cut down center of 1 flat side, cutting to within 1 inch of bottom. Open roast, and sprinkle evenly with 1 teaspoon each of salt and pepper. Chop 2 garlic cloves, and sprinkle evenly over pork. Rub half of the parsley mixture evenly over pork. Fold pork over to close; tie at 1-inch intervals with string. Sprinkle evenly with remaining 2 teaspoons each salt and pepper on outside of pork.
Place orange slices in bottom of a roasting pan with remaining 8 garlic cloves; drizzle with oil. Place pork on top of orange slices; pour 2 cups orange juice over pork.
Bake at 450° for 15 minutes; reduce heat to 325°, and bake, basting every 20 minutes, for 1 hour and 30 minutes. Add tomatoes to pan, and bake 2 more hours or until a meat thermometer inserted into thickest portion registers 150°, basting occasionally with pan drippings.
Remove pork from oven, and coat evenly with remaining parsley mixture. Bake 10 more minutes or until a meat thermometer registers 155°. Remove pork, orange slices, and tomatoes from pan, cover with aluminum foil, and let stand 20 minutes before slicing; reserve drippings in pan.
Combine cornstarch and remaining 1/4 cup orange juice; stir into drippings. Bring mixture to a boil, and cook, stirring constantly, 1 minute or until thickened. Pour sauce through wire-mesh strainer, if desired. Serve pork with tomatoes, orange slices, and sauce. Garnish, if desired.