Using a large stockpot or dutch oven, heat the olive oil over medium heat. Add the sliced leeks and chopped potatoes and cook until the vegetables have begun to soften and brown slightly, about 5 to 10 minutes.
Add the chicken or vegetable stock and bring to a boil. Reduce the heat to low and simmer for about 25 to 30 minutes or until the vegetables are tender and soft.
Turn off the heat and, using your immersion blender, blend the soup until smooth; I like a little chunkiness in my soup so I don't blend it all completely. Add in the heavy cream and season with salt, pepper and the lemon juice.
Garnish with the mushrooms (if using), crème fraiche and minced chives. Serve and enjoy!