Heat butter or oil in a medium soup pot over medium heat. Add chopped onions, carrot and celery and sauté until tender, about 5 minutes. Add garlic, salt and pepper and cook until fragrant, 1–2 minutes. Add milk and water and stir to combine.
Whisking continuously, sprinkle in potato flour and stir until smooth and to the desired thickness. Add your favorite seasonings, if desired.