In a 3.5 to 5 quart slow cooker combine mushrooms, onion, and garlic.
Top with chicken.
In a medium bowl mix broth, tomato paste, wine, basil, salt and pepper. Pour over chicken.
Cover and cook on low for 5-6 hours, or on high for 2.5 -3 hours.
Transfer chicken and vegetables to serving platter.
If using low heat setting, turn to high heat setting.
In a small bowl, mix cornstarch and cold water, stir into juices and cover and cook until thickened (about 15 minutes).
OR
Put juices from slow cooker into small sauce pan, stir in cornstarch mixture and cook over medium high heat until thickened (about 4 minutes).