For mustard butter - stir together all ingredients with 1/4 teaspoon each of salt and pepper.
Bring lentils, water, and 3/4 tsp. salt to a boil in a heavy medium saucepan. Reduce heat and simmer, uncovered, until lentils are just tender, 20-25 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 thoroughly. Cook leeks in butter in a heavy medium skillet over medium-low heat, stirring occasionally, until softened, about 6-8 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 (1/2 to 1 tbsp.) and salt/pepper to taste. Remove from heat and cover to keep warm.
For salmon - Pat salmon dry and sprinkle with 1/2 tsp. salt and 1/4 tsp. pepper. Heat butter in a large nonstick skillet over medium-high heat until foam subsides. Saute salmon, turning once, until golden and just cooked through, 6-8 minutes total.
Serve salmon, topped with remaining mustard-butter, over lentils.