Peel onion and cut lengthwise into 8 wedges. Rinse celery and tomatoes. Trim and discard ends of celery and cut into 3-inch lengths (reserve leaves for garnish); core tomatoes and cut in half lengthwise.
Rinse chicken and pat dry. Sprinkle lightly all over with salt and pepper.
Pour oil into a 10- to 12-inch nonstick frying pan over high heat, tilting to coat bottom. When oil is hot, add chicken and turn as needed to brown lightly on both sides, 4 to 5 minutes total. Transfer to a rimmed plate.
Add onion, celery, and tomatoes to pan; turn vegetables occasionally until lightly browned, 3 to 5 minutes total. Return chicken and any accumulated juices to pan and add broth and thyme.
To Freeze: Freeze in freezer safe container.
To Reheat: Heat in a saucepan until sauce is simmering. Simmer for about 15 mins from defrosted or until chicken is cooked through.