Preheat oven to 450°.
Sprinkle both sides of chicken breasts with tarragon, salt, and 1/4 teaspoon pepper; set aside. Cook onions in boiling water 8 minutes; drain well. Cool to touch; peel. Place 1/4 teaspoon pepper, onions, and remaining ingredients in a shallow baking dish; toss to coat. Arrange chicken on top of vegetable mixture. Bake at 450° for 20 minutes. Reduce oven temperature to 375°; bake an additional 20 minutes.