Slow-Cooker Beef Stew

1 ½ pounds red potatoes, cubed
6 medium carrots, cut into 1 inch slices
1 tablespoon onion powder
3 celery ribs, coarsely chopped
3 tablespoons all-purpose flour
1 ½ pounds beef stew meat, cut into 1 inch cubes
3 tablespoons oil
1 can (14 ½ ounces) diced tomatoes, undrained
1 cup chicken broth
1 teaspoon ground mustard
½ teaspoon salt
½ teaspoon pepper
½ teaspoon dried thyme
Layer the potatoes, carrots and celery in a slow cooker. Sprinkle with onion powder. Place flour in a large resealable plastic bag. Add stew meat; seal and toss to coat evenly. In a large skillet, brown meat in oil. Place over vegetables.
In a large bowl, combine the tomatoes, broth, mustard, salt, pepper and thyme. Pour over beef. Cover and cook on high 1 ½ hours. Reduce heat to low; cook 7-8 hours longer or until the meat and vegetables are tender.