In a Dutch oven or a soup large pot (3 quart or larger), melt the butter over medium-high heat.
Add the sliced leeks and saute for 5 minutes. Add the celery and carrots. Saute for another 5 minutes, until the vegetables begin to soften and sweat.
Add the potatoes and garlic, and saute another for 4-5 minutes.
Add the oregano, thyme, parsley, salt, pepper, and broth.
Bring the soup to a boil, then cover and reduce to a simmer. Simmer the soup for about 25 minutes, or until the veggies are soft.
Remove the soup from the heat source. Use an immersion blender to puree the soup until smooth.
Spoon the sour cream into a small bowl. Whisk 2 tablespoons of hot soup into the sour cream.
Stir the (now tempered) sour cream mixture into the soup. Serve warm.
Notes
*Leeks are usually very dirty, so I recommend soaking them, after they’re sliced, in a bowl of water. After a few minutes of soaking, drain the water and rinse the leeks with fresh water.If you don't have sour cream on hand, use heavy cream. Simply substitute 1/2 cup of cream in place of the sour cream.