1. Make stew: In a large ziplock bag, mix flour, salt, pepper, garlic powder, onion powder, thyme and sage. Add chicken; toss until pieces are coated.
2. Warm 1 Tbsp. oil in a large pot over medium heat. Shake excess flour mixture from chicken (reserve leftover flour), add chicken to pot and cook, stirring, until browned, about 10 minutes. Remove to a plate. Warm remaining oil in pot. Add carrots, celery and onions; sauté for 6 minutes. Sprinkle with reserved flour; sauté for 2 minutes. Pour in broth; bring to a boil, stirring. Reduce heat to low; simmer for 10 minutes, stirring occasionally. Return chicken to pot, stir in half-and-half; simmer for 5 minutes.
3. Make dumplings: Combine both flours, baking soda, salt, thyme and sage. Stir in buttermilk just until a sticky dough forms.
4. Stir peas into chicken mixture. Drop in tablespoonfuls of dough, cover; simmer dumplings until cooked through, about 15 minutes. Serve immediately.