Preheat oven to 475 Degrees Fahrenheit (246 Degrees Celsius).
Peel and wash potatoes and carrots.
Chop potatoes, carrots, and onion. Place on a baking tray.
Coat vegetables with oil, salt, and pepper.
Add garlic and rosemary.
Bake for 40 to 45 minutes or until vegetables are soft.
Place baked vegetables in a pot with water.
Puree and continuously add water until you have the desired texture and consistency.
Reheat soup on the stove and serve.