In large stockpot over medium-high heat, combine chicken stock, beef stock, white wine, water, bay leaf, thyme, parsley and garlic. Season chicken legs with salt and pepper; add to pot. Also place in pot potatoes, Brussels sprouts, celery and parsnips. Bring to a boil over high heat, cover and decrease heat to a gentle simmer. Cook until vegetables are almost tender, about 15 minutes.
Use tongs to gently move ingredients around a bit. Continue to simmer, partially covered, until chicken is almost cooked through, about 5 minutes more. Add green beans and leeks and continue cooking until chicken is completely cooked and vegetables are tender, about 7 minutes.
To serve, ladle broth into soup bowls. Serve with slices of baguette. Dice chicken and vegetables on plate. Ladle over broth in soup bowls. Serve hot, accompanied by mustard, pickles and horseradish.