Place raw chicken breasts, chicken broth, broken celery ribs, and chopped onion into your slow cooker. Secure the lid and cook on low for 3-4 hours or on high for 2-3 hours until the chicken is tender.

Once cooked, carefully remove the celery and onion. Shred the chicken using two forks and return the chicken to the broth, keeping the slow cooker covered to retain warmth.

In a separate bowl, whisk together flour, baking powder, and salt. Gradually stir in milk to create a shaggy dough; it should be slightly sticky.

On a floured surface, roll out the dough to about 1/4-inch thickness. Use a knife or pizza cutter to slice the dough into strips or squares.

Stir the dumplings into the hot broth. Cover the slow cooker and let them cook for an additional 15 minutes; stir gently to prevent sticking.

Continue to cook until the dumplings are tender and fluffy, about another 15 minutes. Season with salt and pepper before serving to taste.