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1
Trim and discard root ends and coarse tops from leeks. Cut leeks in half lengthwise and rinse under running water, flipping layers to flush out grit; drain, then thinly slice crosswise.
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2
Rinse fennel. Trim off and discard root ends and stalks; reserve 2 or 3 sprigs of feathery green tops for garnish, and finely chop enough of the remaining feathery leaves to make 3 tablespoons (discard any remaining greens). In a bowl, mix chopped greens with chives; cover and chill. Chop fennel heads.
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3
In a 6- to 8-quart pan over medium heat, melt butter. Add leeks and chopped fennel heads, cover, and stir occasionally until vegetables are very limp, 10 to 12 minutes.
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4
Add broth and bay leaf to pan. Bring to a boil over high heat. Scrub potatoes and cut into 1/2- to 3/4-inch cubes.
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5
Add potatoes to broth mixture and return to a simmer; reduce heat, cover, and simmer, stirring occasionally, until potatoes are tender when pierced, 15 to 20 minutes.
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6
Cut salmon into strips 2 to 3 inches long and 1/2 inch wide; put in a bowl.
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7
In a separate bowl, whisk milk, flour, 1/2 teaspoon salt, and 1/8 teaspoon pepper until smooth. Add to potato-broth mixture and stir over high heat until boiling, about 5 minutes. Add more salt and pepper to taste.
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8
Serve soup from pan, or pour into a tureen. Garnish with reserved fennel sprigs. Offer with smoked salmon and chive mixture to add to taste.
