Season chicken thighs with salt and pepper on both sides. Place in the bottom of a 6-quart slow cooker.
Layer vegetables over chicken: chopped onion, red and green bell pepper strips, sliced mushrooms, and minced garlic.
Whisk together crushed tomatoes, tomato paste, chicken broth, oregano, and basil in a mixing bowl until well combined.
Pour tomato mixture over chicken and vegetables in slow cooker, ensuring chicken is mostly covered by the sauce.
Cover and cook on LOW for 6-7 hours or on HIGH for 3-4 hours, until chicken is fork-tender.
Remove bay leaf and stir in cornstarch mixture during the last 30 minutes of cooking to thicken sauce.
Serve hot garnished with fresh basil over pasta, rice, or mashed potatoes.