In a Dutch oven over medium heat, melt butter. Add leeks and onion; cook until translucent about 15 minutes. Add potatoes, broth, freshly ground black pepper, sea salt, tarragon, basil and lemon juice. Bring to a boil then reduce heat to a simmer; cook for 30 minutes.
When time is up, turn off the heat, give a good stir and let it cool off a bit before puréeing with an immersion blender until smooth. Stir in heavy cream and adjust seasoning if needed. Transfer the soup to a large bowl and cover with wax paper. When cool enough cover with a lid (leave the wax paper to prevent skin formation) and transfer to the refrigerator for 24 hours or until chilled.
Just before serving, add a dollop of sour cream garnish with crumbled cooked bacon and fresh chives.
Note: Submerge peeled potatoes in cold water to prevent from browning.