Trim the dark green tops from the leeks. Slice the white and light green sections lengthwise and rinse each layer thoroughly under cold running water. Pat dry and slice thinly to remove any grit.

In a large heavy-bottomed soup pot, melt butter over medium-low heat. Add the leeks and sliced onion, stirring frequently for about 10 minutes, until they become soft and fragrant. Avoid browning them.

Stir in the cubed Yukon Gold potatoes, then pour in the chicken broth, stirring gently to combine. Increase the heat slightly to bring the mixture to a light simmer.

Cook uncovered for 20 to 25 minutes, stirring occasionally until the potatoes are very soft.

Remove the pot from heat and let it cool for several minutes. Use an immersion blender to puree the soup or carefully transfer batches to a blender until completely smooth.

Strain the blended soup through a fine mesh strainer into a clean bowl, discarding any fibers from the leeks.

Stir in the heavy cream, salt, and white pepper. Adjust seasonings as needed.

Allow the soup to cool before transferring it to the refrigerator. Chill for at least 4 hours or preferably overnight.