Sprinkle the chicken with salt and pepper and dredge in the flour. Melt 2 tablespoons of the butter in a Dutch oven over medium-high heat. Brown the chicken, working in batches, and then transfer to a plate.
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.
Cool and store for a make-ahead meal. Bring to room temperature before reheating over medium heat.