How to Make It
Set a medium strainer lined with a coffee filter over a small saucepan. Put the grated horseradish in the filter and gradually pour the boiling water over it.
In a large saucepan of boiling water, blanch the leeks, carrot and fennel for 1 minute. Drain in a colander and transfer the vegetables to a bowl.
In a large skillet, heat the 2 tablespoons of olive oil. Add the leeks, carrot and fennel and cook over moderate heat, tossing, until hot. Stir in the mustard and season with salt and pepper. Remove from the heat.
In a medium saucepan, bring the remaining 1 1/2 cups of olive oil to a simmer over moderate heat. Season the salmon fillets with salt and pepper and add to the oil. Gently simmer the fillets over low heat for 4 minutes. Turn and simmer until just cooked, about 3 minutes longer.
Meanwhile, gently reheat the horseradish broth and season with salt. Rewarm the vegetables. Pour the broth into 4 shallow bowls. Spoon the vegetables into the bowls and top with the fillets. Sprinkle with fleur de sel and pepper and serve.