Preheat oven to 425 degrees F. In a large skillet, melt butter over medium-high heat. Add chicken, sweet pepper (if using), and shallots. Cook for 4 to 5 minutes or until chicken is light brown and no longer pink, stirring frequently. Stir in flour, salt, tarragon, and black pepper. Add milk all at once. Cook and stir until thickened and bubbly. Stir in wine and peas; heat through. Keep warm while preparing pastry.
For pastry, unfold and roll puff pastry sheet into an 11-inch square. Cut into four equal squares. Transfer the hot chicken mixture to four 8-ounce individual casseroles. Place a pastry square on top of each casserole. Flute edges of pastry and cut slits in the top for steam to escape.
Bake, uncovered, for 20 minutes or until crust is puffed and golden. Makes 4 servings.