In a large stock pot, heat olive oil and sauté onion until tender, about 5 minutes. Add garlic and cook for 2 minutes. Add potatoes,vegetable broth, and water to the pot. Bring to boil and reduce to medium heat. Let potatoes cook until tender, about 20 minutes. With a slotted spoon, remove 2 cups of the potatoes. Set aside. Puree the remaining potatoes with an immersion blender or put potatoes in a blender or food processor.

In a small sauce pan, melt butter. Stir in flour and cook for 2 minutes. Whisk in milk, bring to a boil and simmer until thickened. Slowly stir mixture into large stock pot with the pureed potatoes. Bring to a boil and reduce heat to a low simmer. Stir the 2 cups of potatoes back into the soup.

Stir in feta cheese and fresh rosemary. Season with salt and pepper, to taste. Cook for an additional 5 minutes. Serve warm.