Heat oil in skillet. Brown chicken breasts over med high heat until browned. Season with salt and pepper. Add chopped onion and saute until onions are softened. Add remaining ingredients. Bring to a boil. Reduce heat to simmer, cover and allow to cook for at least 45 minutes. I find it best to transfer the mixture (0nce it has come to a boil) to a crock pot and cook it on low for 8 hours. It just melts in your mouth. Serve over pasta.