Bake shells at 400°F following package directions; keep warm.
Meanwhile, place chicken breasts in small saucepan with enough water to cover. Simmer for 20 minutes, until chicken is cooked through. Drain. When cool enough to handle, cut into 1/2-inch cubes.
Heat butter in large nonstick skillet over medium-high heat. Add mushrooms and scallion; saute for 7 minutes or until mushrooms are tender. Add chicken, tarragon, Mornay Sauce, mustard, salt, black pepper and red pepper; cook for 4 minutes or until heated through. Divide over individual pastry shells. Serve at once.