Saute Celery, Carrot and Onion in Oil until soft, about 7 minutes. Sprinkle 1 tsp Kosher salt over veggies while cooking.
Sprinkle Flour over veggies and stir 2 minutes. Gradually whisk in broth, then milk.
Add Chopped Potatoes and bring soup to a boil. Reduce heat and simmer until potatoes are tender, 20-30 minutes.
Turn off heat and slowly add the cheese. Stir until melted and smooth. Make sure the soup is not too hot when you add the cheese or it will curdle and separate. Add additional salt, if needed.
Serve soup with additional Cheese, Ham/Bacon and Sour Cream/Greek Yogurt, optional