Add a drizzle of EVOO to the pot along with the remaining butter, onions, garlic, celery, fennel and leeks. Sprinkle the vegetables with some salt and pepper and cook until lightly browned, 7 to 8 minutes. Stir in the bay leaf and wine, return the chicken to the pot and cook, partially covered, over a low, rolling simmer, 30 minutes.
Stir in the creme fraiche, reduce to a simmer and continue to cook, 30 minutes longer. If the liquid gets too low, add the chicken stock. Transfer the chicken to a warm platter, and add the mustard and chopped tarragon at the last minute. Serve with warm baguette.