This is a great excuse to eat some fancy mashed potatoes as an entree.
To easily clean the leeks after coarsely chopping them, submerge them in a large bowl of water and aggressively agitate to release as much dirt as possible. Discard the water and repeat once or twice until the water runs clear.
In a 4-quart soup pot over medium heat, melt butter.
Add leeks, celery, kosher salt, and black pepper, and cook until leeks are wilted (about 10 minutes).
Add garlic, and cook until fragrant (about 1 minute).
Add chicken stock, russet potatoes, thyme, and bay leaf. Bring to a boil, reduce to a simmer, and cook until potatoes are tender (about 20 minutes).
Discard bay leaf.
Using an immersion blender, blend until smooth.
Add heavy cream, and simmer until thickened.
Season to taste with kosher salt and black pepper.
Garnish with nutmeg and dill (chives work well too).