Preheat oven to 300°F.

Pat lamb dry. Brush all over with 1 tablespoon of oil. In small bowl, combine oregano, thyme, rosemary, salt and pepper; rub all over roast.

Heat remaining olive oil in large ovenproof Dutch oven or casserole dish set over medium high heat. Brown roast all over. Transfer roast to plate. Add onions, carrots, celery, garlic, and mushrooms to pot. Cook, stirring, for 10 minutes or until softened and any excess liquid released from the mushrooms has evaporated. Pour in tomatoes, wine, and bay leaves; bring to a boil. Return meat to pot.

Bake in center of preheated oven for two and a half to three hours, or until meat is extremely tender.

Remove meat to cutting board. Tent with foil. 

Slice lamb. Serve topped with sauce.

Skim off as much fat from juices as possible. Remove bay leaves. Set the juices over medium-high heat. Bring to a boil and boil for 15 minutes. Dissolve potato starch in 3 tablespoons of water. Whisk into sauce. Boil gently for 5 minutes or until thickened.