Prep Leeks: Chop off the tough dark green tops and root ends from the leeks. Slice the remaining white and light green parts lengthwise, fan out under cold running water, and rinse thoroughly to remove any grit. Pat dry and slice into half-moons.

Sauté Base: Melt the butter in a large pot or Dutch oven over medium heat. Add the sliced leeks and cook gently, stirring occasionally, for about 5-7 minutes until softened and sweet, but not browned.

Cook Potatoes: Add the peeled and chopped potatoes to the pot. Stir and cook for another couple of minutes.

Simmer: Pour in the stock. Add the optional bay leaf if using. Bring the mixture to a gentle simmer, then reduce the heat, cover partially with a lid, and let it bubble away until the potatoes are very tender and easily pierced with a fork (approx. 15-20 minutes).

Blend: Remove and discard the bay leaf (if used). Carefully blend the soup until smooth. You can use an immersion blender directly in the pot (start slow to avoid splashes) or transfer the soup in batches to a regular blender (do not overfill, be cautious with hot liquid).

Finish: Return the blended soup to the pot if necessary. Stir in the heavy cream or milk.

Season: Season generously with salt and freshly ground black pepper. Taste the soup and adjust the seasoning as needed.

Serve: Ladle the hot soup into bowls. Add an optional squeeze of fresh lemon juice just before serving, if desired. Pairs wonderfully with crusty bread.