Peel and dice the onion, potatoes and carrots.
Melt butter over medium heat in a large pot. Add the onion and cook until translucent, about 2 minutes. Add the potatoes and carrots. Continue to cook over medium heat for about 5 minutes.
Add the vegetable stock (we love this one) and increase heat to high in order to bring the soup to a boil. Reduce heat to low or medium-low, and simmer, covered, for 25 minutes.
Once the potatoes and carrots are soft enough to come apart easily with a fork, puree the soup with a hand blender or by batches in a countertop blender or food processor, until it reaches a cream-like consistency. If you didn't use a hand blender, return the creamy carrot potage to the pot, over low heat. Add more stock for a more liquid consistency, or leave as is for a creamier soup. Add sea salt and pepper to taste.
Serve as is or with fresh parsley and a drizzle of crème fraîche or heavy cream.