- With the rack in the middle position, preheat the oven to 450°F (230°C). Line a baking sheet with aluminum foil.
- In a non-stick skillet over medium-high heat, soften the vegetables in the butter for 5 minutes. Sprinkle with the flour and cook for 1 minute, stirring constantly. Add the milk and broth. Bring to a boil while stirring. Add the chicken. Simmer for 10 minutes, stirring occasionally, until the béchamel sauce thickens and the vegetables are tender. Add the peas. Season with salt and pepper.
- Place the baguette halves side by side on the baking sheet, cut side up. Cover with the chicken mixture and bake for 8 minutes. If desired, finish under the broiler for 2 to 3 minutes until golden brown.