1. Sprinkle chicken with 1/4 teaspoon salt and pepper. Heat a large nonstick skillet over medium-high heat. Add oil to pan; swirl to coat. Add half of chicken to pan; cook 5 minutes on each side or until lightly browned. Place chicken in a 5-quart electric slow cooker coated with cooking spray. Repeat procedure with remaining chicken. Place mushrooms on top of chicken.
2. Add garlic, onion, and bell peppers to pan; sprinkle vegetables with remaining 1/4 teaspoon salt. Reduce heat to medium, and cook 5 minutes or until vegetables are crisp-tender, stirring often. Add wine, scraping pan to loosen brown bits. Cook 1 minute. Stir in flour. Stir in oregano, thyme, and tomatoes.
3. Pour tomato mixture over mushrooms in slow cooker. Cover and cook on LOW for 3 hours or until chicken is very tender. Serve over fettuccine. Sprinkle with additional thyme, if desired.