Cut chicken into 1-in. cubes; brown in 1 tablespoon oil in a large skillet. Transfer to a 3-qt. slow cooker. In the same skillet, saute the mushrooms, onion, zucchini and green pepper in remaining 1 Tbsp. oil until crisp-tender; add garlic; cook 1 minute longer. , Place in slow cooker. Add the tomatoes, tomato paste, water and seasonings. Cover and cook on low for 4-5 hours or until the meat is no longer pink and vegetables are tender. If desired, top with chopped fresh thyme.