For the horseradish cream, mix the sour cream, Greek yogurt, horseradish and white pepper in a small bowl. Cover and refrigerate.
For the short rib stew:
Rinse the dried mushrooms in cold water to wash off any debris or dirt. Chop into small pieces and set aside.
Mix together the flour, onion powder, salt and pepper in a small bowl. Sprinkle over short ribs and toss to coat. Heat 3 tablespoons olive oil in a large Dutch oven over medium-high heat. Working in batches, sear the short ribs on all sides until a crust forms, about 5 minutes per side. Add oil between batches as needed. Remove the short ribs to a plate.
Add the shallots to the Dutch oven and saute until slightly translucent. Add the dried mushrooms and the creminis, and saute for 5 minutes. Deglaze the pan with the red wine and cook until reduced by half. Add the beef stock, barley, rosemary and the browned short ribs. Bring to a boil, then lower to a simmer. Cook, covered, for 2 hours, checking the liquid level occasionally; if it falls below the beef, add stock to cover.
Carve out bread bowls: Insert a knife into the top each loaf and cut in a circular motion to remove the tops. With your hand or a spoon, scoop out the insides of the loaves; reserve this for dipping.
Ladle the hot stew into the bread bowls. Top each with a dollop of horseradish cream and garnish with parsley. Serve on plates with the reserved bread pieces on the side.