For the chicken: Preheat the oven to 425 degrees F. When hot, put a cast-iron skillet in the oven to preheat, about 5 minutes.
Dry the chicken with paper towels. In a small bowl, mix together the salt, pepper and garlic powder. Generously season the chicken inside and out with the salt mixture. Stuff the chicken cavity with the thyme, rosemary, garlic and onion.
Slice the baguette on the bias into five 2-inch-thick slices. Spread some butter on one side of each slice. Carefully remove the hot skillet from the oven and arrange the baguette slices buttered-side down in the center of the skillet. Place the chicken on the baguette slices, making sure all of the bread is covered by the chicken. Roast until the chicken is golden brown, an instant-read thermometer reads 165 degrees F and the juices run clear, about 1 hour and 15 minutes. Transfer the chicken to a cutting board and tent with foil. Let rest for 10 minutes before carving. Remove the baguette slices from the skillet and reserve.
For the pan sauce: While the chicken is resting, put the skillet over medium-high heat. Pour in the wine and scrape with a wooden spoon to remove all the brown bits. Add the chicken stock and cook until the liquid is slightly reduced, about 2 minutes. If any juices from the chicken have collected on the cutting board, pour them into the skillet now. Add in the butter piece by piece, waiting for each to melt before stirring in the next piece. Stir in the rosemary and thyme. Season with salt and pepper.
For serving: Carve the chicken and serve with the baguette slices, pan sauce, Dijon mustard and Bistro Salad.
Combine the lettuce and herbs in a medium bowl. In a glass jar or plastic container with a tight-fitting lid, combine the olive oil, vinegar, shallots, mustard, salt and pepper. Shake to combine. Pour over the salad and toss. Serve immediately.