Sauté the Vegetables: Place the butter or oil in a large pot or Dutch oven over medium heat.
Once hot, add the sliced onion, chopped carrots, and potato.
Season generously with salt and pepper and cook, stirring occasionally, for about 15 minutes, until the vegetables have softened slightly.
Simmer the Soup: Pour in the vegetable stock, bring to a boil, then reduce the heat to a simmer.
Cook for 15 to 20 minutes, or until the carrots and potato are very tender and easily pierced with a fork.
Blend Until Smooth: Remove the pot from the heat. Use an immersion blender to purée the soup directly in the pot until it's completely smooth. Alternatively, let the soup cool slightly, then carefully transfer it in batches to a standard blender and blend until velvety.
Finish and Serve: Return the puréed soup to the pot if necessary.
Taste and adjust the seasoning. If the flavor seems a bit flat, stir in the optional sugar to bring out the carrots' natural sweetness. If serving hot, gently reheat without boiling. If serving cold, chill for at least 2 hours. Just before serving, stir in the cream or sour cream, if using.
Garnish with fresh parsley and enjoy.