Sprinkle chicken with 1/4 teaspoon salt and 1/4 teaspoon pepper. Meanwhile, in a large skillet, heat oil over medium-high heat. Add chicken; cook and stir until no longer pink, 6-8 minutes. Transfer to a 6-qt. slow cooker. , In same skillet, cook celery, carrots and onion until tender, 6-8 minutes. Add garlic, tomato paste and remaining salt and pepper; cook 1 minute. Stir in flour; cook 1 minute longer. Whisk in 2 cups chicken broth; cook and stir until thickened. Transfer to slow cooker. Stir in bay leaves, thyme and remaining chicken broth. , For dumplings, whisk together flour, baking powder, salt and pepper in a large bowl. Stir in milk and butter to form a thick batter. Drop by 1/4 cupfuls over chicken mixture. Cook, covered, on low until bubbly and dumplings are set, 6-8 hours. Discard bay leaves. Remove insert and let stand, uncovered, for 15 minutes.