In a stockpot or Dutch oven, add broth, celery, onion, and chicken breasts.
Bring to a boil, then reduce heat and simmer, partially covered, for about 15 minutes, until the chicken breast is cooked through. While the chicken is cooking, prepare the dumplings.
In a medium mixing bowl, whisk together flour, baking powder, and salt. Stir in the milk just until combined into a shaggy dough.
Turn the dough out onto a floured surface and roll to about ¼ inch thickness.
Use a pizza cutter to cut the dough into strips or squares about 1 to 2 inches in size.
Once the chicken is cooked through, remove it to a cutting board. Remove the onion chunk and celery pieces and discard.
Add the dough strips to the simmering broth and simmer for about 20-30 minutes, stirring occasionally, allowing the dumplings to cook and the broth to thicken to a gravy-like consistency.
While the dumplings are simmering, shred or chop chicken and return to the pot.
Add salt and pepper to taste, then serve warm.