After you chop and sauté the lamb and veggies, the stew needs a good long simmer. While it simmers, you can curl up under a blanket and wait for the delicious aroma fill your house!
Brown lamb pieces in batches, adding additional oil, if necessary. Remove from pot and set aside. Saute mushrooms until they're browned and have released their moisture. Remove from pot and set aside with the lamb.
Saute onions, carrots, celery, salt, pepper and herbes de provence. Stir in tomatoes and 2 cups of the beef stock, along with reserved lamb and mushrooms. Bring to a boil, reduce heat, cover and simmer for 1-1/2 hours.
Stir in sweet potatoes and simmer covered for an additional 30 minutes. Whisk together remaining 1/2 cup beef stock and flour. Stir in stock and flour mixture. Continue cooking until stew has thickened.