Melt butter in medium saucepan over medium heat. Sauté onion in melted butter until soft, about 5 minutes, then add garlic and sauté, stirring constantly, about 1 minute, or until deeply aromatic.

Sprinkle flour over and cook into a deep, golden-brown roux, about 2-5 minutes or so. Add in stock and whisk until smooth. Bring to a boil then lower heat to medium-low and simmer until a little thickened, about 5 minutes.

Add in potatoes and 1/2 cup milk and whisk until smooth. Heat through. Add in additional milk until your desired consistency (I used a full 1 cup milk).

Blend either with immersion blender or in batches in a blender (be careful! Keep a towel over the top and make sure the lid is on tight. Always go with smaller than larger batches. The last thing you want is a nice spray of hot potato soup from the blender.. This is why I recommend an immersion blender), until desired consistency.