Place potatoes in a shallow roasting pan and drizzle with 1 tablespoon olive oil. Sprinkle with pepper; stir to coat. Bake for 25 minutes, or until potatoes are browned. Reserve 1 cup of roasted potatoes.
In a 3 litre saucepan heat remaining oil; saute onions for 5 minutes. Add potatoes and garlic and stir in broth and water. Bring to a boil, reduce heat and simmer, uncovered, for 20 minutes.
Spoon half of broth mixture into a blender; blend until nearly smooth. Repeat with remaining mixture; return all to pot. Stir in milk and season with salt to taste. Ladle into bowls and top with reserved roasted potatoes.