Season chicken with salt and pepper. Heat oil in a large skillet over medium-high heat. Add chicken and cook until golden. Transfer to a lightly greased slow cooker.

Add celery, carrots, onion and garlic to same skillet and cook until tender, about 5 minutes. Transfer to slow cooker with chicken.

In a large bowl, whisk together flour, broth, thyme and salt and pepper until smooth. Stir in bay leaves. Pour mixture over chicken in slow cooker.

For the dumplings, whisk together flour, baking powder, salt and pepper and in a large bowl. Stir in milk and melted butter (batter will be thick). Drop by 1/4 cupfuls over chicken mixture. Cover and cook on low until chicken is bubbly and dumplings are set, 6-8 hours; discard bay leaves. Turn off slow cooker and let stand, uncovered, for 15 minutes before serving.