Sprinkle chicken thighs with salt and pepper. In 4-quart Dutch oven, heat oil over medium-high heat until hot. Add chicken in batches; cook 4 to 6 minutes or until browned on both sides. Remove chicken from Dutch oven.
Add onions to Dutch oven; cook 3 to 4 minutes, stirring frequently, until tender. Add chicken and broth; reduce heat to low. Cover; cook 25 to 30 minutes or until juice of chicken is clear when center of thickest part is cut (180°F). Remove chicken; place on platter. Cover with foil to keep warm.
In small bowl, mix milk and flour until smooth. Beating with wire whisk, add flour mixture to cooking juices in Dutch oven. Increase heat to medium. Add peas and tarragon; cook about 5 minutes, stirring frequently, until peas are tender and sauce has thickened slightly. Pour mixture over chicken.