Heat olive oil in a large skillet or Dutch oven over medium heat. Brown chicken thighs on both sides and remove to a plate.
Sauté onion and garlic in the same pan until soft. Add carrots and celery; cook for 5 minutes.
Add butter and flour to the pan. Stir to create a paste and cook for 2 minutes.
Whisk in chicken broth, milk, and all gravy seasonings. Bring to a simmer until thickened.
Dice the cooked chicken and return to the skillet. Cover and simmer.
In a bowl, mix all dry dumpling ingredients. Stir in milk and melted butter until a soft dough forms.
Drop spoonfuls of dough onto the simmering stew. Cover tightly and steam for 15 minutes without lifting the lid.
Notes
Keep the lid on while the dumplings steam to ensure they remain light and fluffy. If the dumplings are too large, they will take longer to cook through.