Potage Parmentier (Potato Leek Soup)

neutral oil, such as canola or grapeseed oil

to 5 medium russet potatoes (1 pound), peeled and roughly chopped

large leeks (1 pound), cleaned and thinly sliced

vegetable stock (or light chicken stock)

Kosher salt,

to 2 tablespoons freshly squeezed lemon juice

heavy cream

crème fraiche

minced parsley or chives