Sauté Onion (2) in Butter (4 tablespoon) over medium heat for 15 minutes, or until tender and just beginning to turn color.
Add Potato (4) and water to cover, about 2 1/2 cups. Simmer until tender, about 20 minutes.
In a separate pot, melt the butter over medium heat. Add All-Purpose Flour (2 tablespoon) and stir with a wooden spoon until a light brown paste forms, a minute or so.
Slowly add Milk (3 cup), whisking constantly. Bring milk mixture to a gentle simmer, stir, then keep warm.
When potatoes are tender, add milk mixture to the pot. Purée using a blender. Season with Salt and Pepper (to taste). Bring soup to a gentle simmer, stirring to ensure it isn't sticking to the bottom ofthe pot.
Garnish with chopped Fresh Herbs (to taste). When reheating the soup, thin with water as needed and adjust seasoning to taste.