In a saucepan, sauté onion, celery and carrots in butter until softened but not browned. Add tomatoes and a small amount of chicken broth. Simmer for 15 minutes. In a Dutch oven or stock pot, combine sautéed vegetables, remaining chicken broth and rice. Season with salt, thyme and pepper. Simmer 20 to 30 minutes. Serve garnished with parsley. If desired, blend a few cups of soup and add back to the mixture for a thicker soup.