Despite the fact that salmon seems to be everywhere these days, this dish is something very special. Mustard-herb butter gives a lemony pop to the meatiness of the fish, while French green lentils add a unique texture that regular brown lentils just don't have.
Make mustard-herb butter:
Stir together all ingredients with 1/4 teaspoon each of salt and pepper.
Bring lentils, water, and 3/4 teaspoon salt to a boil in a heavy medium saucepan, then reduce heat and simmer, uncovered, until lentils are just tender, 20 to 25 minutes. Remove from heat and let stand 5 minutes. Reserve 1/2 cup cooking liquid, then drain lentils.
While lentils cook, chop leeks, then wash . Cook leeks in butter in a heavy medium skillet over medium-low heat, stirring occasionally, until softened, 6 to 8 minutes.
Add lentils with reserved cooking liquid to leeks along with 3 tablespoons mustard-herb butter and cook, stirring, until lentils are heated through and butter is melted. Add lemon juice and salt and pepper to taste. Remove from heat and keep warm, covered.
Saute salmon while leeks cook:
Pat salmon dry and sprinkle with 1/2 teaspoon salt and 1/4 teaspoon pepper (total).
Heat butter in a large nonstick skillet over medium-high heat until foam subsides, then sauté salmon, turning once, until golden and just cooked through, 6 to 8 minutes total.
Serve salmon, topped with remaining mustard-herb butter, over lentils.
·Mustard-herb butter can be made 1 day ahead and chilled, covered. Soften at room temperature before using (1 hour).
·Lentils can be cooked (but not drained) 1 day ahead and chilled in cooking liquid, covered (once cool).