Step 1

Set a wire rack over a large sheet pan and set it near your work space.

Step 2

Cook the bacon in a large skillet over low heat until crisped; transfer to the prepared sheet pan to drain. Pour off all but 2 tablespoons of fat from the skillet; increase the heat to medium-low.

Step 3

Add the onion to the skillet and cook for about 3 minutes, stirring, until softened. Add the celery root, cover the skillet and cook, stirring occasionally, until tender, about 15 minutes. Turn off the heat.

Step 4

Combine the broth or clam juice and the water in a large, heavy saucepan over medium-high heat. Bring to a boil and add the potatoes, then reduce the heat to medium and cook until the potatoes are tender, 8 to 10 minutes. Use a slotted spoon to transfer them to a platter.

Step 5

Add the salmon to the saucepan; reduce the heat to low and cook gently just until the fish is firm, about 4 minutes. Transfer the salmon to the platter with the potatoes.

Step 6

Increase the heat to medium-low and add the cream to the saucepan and cook, stirring frequently, until the mixture has reduced by about one-quarter, 15 to 20 minutes. Watch closely so the mixture does not boil over.

Step 7

Use a fork to blend the butter and flour in a small bowl to form a smooth paste; whisk it into the saucepan a little at a time until thoroughly incorporated. Increase the heat to medium; cook, stirring, until the soup thickens slightly and loses the raw flour taste, about 3 minutes.

Step 8

Stir in the reserved celery root mixture, potatoes and salmon. Cook for about 2 minutes until warmed through, then stir in the dill, parsley, lemon zest and lemon juice. Season with the salt and white pepper to taste.

Step 9

Divide among individual bowls, crumble some of the bacon on each portion and serve hot.