How to Make It
Preheat the oven to 375°. Line a baking sheet with parchment. On a floured surface, roll out the puff pastry to a 13-by-16-inch rectangle. Using a 5-inch plate, cut out 6 rounds. Transfer to the baking sheet and brush with 2 tablespoons of the cream. Bake for 15 minutes.
In a large saucepan, boil the clam juice and water. Add the potatoes and cook until tender; transfer to a plate. Add the salmon and simmer gently for 4 minutes; transfer to a plate. Add the remaining 1 1/2 cups of cream and simmer over low heat, stirring occasionally, until reduced to 3 cups, about 20 minutes.
Blend the flour and butter; gradually whisk into the simmering sauce until smooth. Cook, stirring, for 3 minutes.
In a large skillet, cook the bacon over low heat until crisp; transfer to a plate. Pour off all but 3 tablespoons of the fat. Add the onion and cook until softened. Add the celery root, cover and cook, stirring occasionally, for 15 minutes.
Stir the celery root, potatoes, bacon, dill, parsley, lemon juice and zest into the sauce; season with salt and pepper. Fold in the salmon and heat through. Spoon the stew into 1 1/4-cup bowls, top with the pastry puffs and serve.