For yogurt sauce: Whisk first 4 ingredients in medium bowl. Season to taste with coarse salt and pepper. DO AHEAD: Can be made 2 days ahead. Cover and chill.
For vegetable stew: Bring 8 cups water to boil in heavy large saucepan. Sprinkle with coarse salt. Add carrots; cook until just tender, about 4 minutes. Using skimmer or large slotted spoon, transfer carrots to large bowl of ice water. Return water to boil. Add turnip; cook until just tender, about 3 minutes. Using skimmer, transfer turnip to bowl with carrots. Reserve cooking liquid.
Melt butter in heavy large pot over medium heat. Add next 7 ingredients. Sprinkle with coarse salt and pepper. Cook until onions are soft, stirring often, about 8 minutes. Add wine; simmer until reduced by half, about 5 minutes. Stir in flour. Add carrots, turnip, beans, spinach, and 2 cups reserved cooking liquid. Bring to simmer; cook until vegetables are heated through, adding more cooking liquid for desired consistency. Season stew to taste with coarse salt, pepper, and lemon juice.
Divide stew among bowls. Spoon dollop of yogurt sauce over and serve.
* A thick yogurt; sold at some supermarkets and at specialty foods stores (such as Trader Joe's and Whole Foods markets) and Greek markets. If unavailable, place regular yogurt in cheesecloth-lined strainer set over large bowl. Cover and chill overnight to drain.
** A spicy North African red chile paste; available at some specialty foods stores and at Middle Eastern markets.