Heat butter and olive oil in a large stock pan over medium heat. Add onions, celery, and carrots and cook until onion is translucent. Add garlic and cook until fragrant.
Stir in flour and thyme and cook for one minute. Stir in chicken stock and then add chicken, salt, and pepper. Reduce heat and simmer while you make the dumplings.
To make the dumplings, whisk together in a bowl flour, salt, baking powder, and thyme. Then stir in butter and milk until combined.
Scoop the dumpling batter using a small cookie scoop on top of the soup. Cover the soup with a tightly sealed lid (no steam holes) and cook for 15 minutes - do not peek.