Arrange slices of one onion in a nine- x 13-inch baking pan. Place chicken onto onion slices.

Sauté second onion in oil. Add remaining ingredients and cook until thickened. Pour sauce over chicken.

Preheat oven to 400 degrees Fahrenheit. Bake, covered, for 30 minutes. Uncover and bake for an additional 15-20. (If chicken is baked covered the entire time, sauce may become very liquidy. To thicken, remove chicken, dissolve three to four tablespoons cornstarch in a bit of water and stir into sauce. Cook, stirring constantly until thickened.)