Heat the oil in a large stockpot or dutch oven over medium heat.
Add the leeks, and cook, stirring occasionally, until the leeks begin to soften, about 10 minutes. Next, add the garlic, potatoes, thyme, salt and pepper, and cook for about 5 minutes.
Add the vegetable broth and bay leaves, and bring to a boil. Reduce the heat to low, and simmer for 30 to 40 minutes, or until the vegetables are tender.
Use an immersion blender to blend until smooth and consistent without any vegetable chunks. Fold in the sour cream and adjust seasoning to your taste.
Serve with fresh watercress and toast, if desired
Notes
Storage: Store any leftovers in an airtight container. They will last about 4-5 days in the fridge.Freezing Instructions: You can also freeze the soup in for up to 3 months. To re-heat, thaw in the fridge overnight and microwave until heated through. Or simply microwave straight from the freezer until heated through.