PREHEAT OVEN TO 350 DEGREES. Place rack at center.
BROWN THE MEAT AND ADD THE VEGETABLES: Heat a 5-6-quart dutch oven over medium high heat for two minutes; add oil. Meanwhile, toss beef with flour, salt and pepper. When oil is very hot, add half of the beef pieces and brown on both sides; remove to a bowl. Brown the other half, adding a little more oil if needed. After turning to brown the second side, add the onion, garlic, celery, carrots, parsnip, turnip, and parsley to the pot. Stir well and cook about 3 minutes or so until very hot.
ADD HERBS, MUSTARD, HORSERADISH, AND WINE: Season with the rosemary and thyme. Stir in the mustard and horseradish and heat through. Add wine and let cook 3-4 minutes, stirring regularly, until wine has reduced by half.
ADD BROTH, BOIL; STIR IN BARLEY: Pour in chicken broth and water, cover, and bring to a boil. Stir in barley. Taste, adjust seasonings, adding salt and pepper to taste. (I added about the same amount as I did the first time through.)
COOK IN THE OVEN: Cover and simmer in the oven for about 2 1/2 hours –give or take–or until meat and vegetables are tender and the stew has thickened, checking midway through to make sure the stew doesn’t need more liquid–broth or water. The liquid should still be about 3/4 of the way up the pot at that point. If it’s not, add some water or broth, not more wine. When everything is done, taste one last time, add a little more liquid if necessary, and re-season to taste.
SERVE HOT in warmed bowls garnished with a small dollop of sour cream or yogurt, if using, and the reserved minced fresh parsley.