Heat the olive oil in a large pot, add the leeks and potatoes, and cook for about 10 minutes until the leeks start to brown and stick to the potatoes.
Add the pumpkin and the parsnip, and continue to cook, stirring a bit, for about another 10 minutes, until the entire mixture is warm and bubbling from the bottom.
Meanwhile warm up the broth in a separate pan. Add the warmed broth to the potato and pumpkin mixture, bring to a boil, and then reduce the heat to a rolling simmer. Simmer the soup until everything is cooked through and well blended, about 20 or 30 minutes. The longer you cook the more thick and rich it will become. I found this time frame to be perfect.
Remove from the stove, add the pepitas and herbs de provence, and give it a quick stir to incorporate.
Ladle the soup into a blender about halfway full. Cover the blender pitcher, and start by pulsing at very low speed, to avoid the hot liquid from bursting out due to high pressure. Gradually increase the speed until you are able to completely purée the soup.
Repeat until all the soup is puréed. Return the soup to the heat just to bring it up to a good temperature for serving.
Taste for salt and pepper before serving. Serve with a chopped parsley and a hint of freshly grated nutmeg on top.