1 medium leek, white and pale-green parts only, cut into 1 \ 2 -inch-thick rounds, rinsed well
1 small Savoy cabbage (about 1 pound), cored and cut into 1-inch-thick wedges
3 tablespoons Dijon mustard
4 salmon fillets (at least 2 1 \ 2 inches wide and about 7 ounces each), with skin
1 \ 4 cup chopped fresh flat-leaf parsley
1 \ 2 lemon
Put the lentils, carrots, garlic, and bay leaf in a medium saucepan; cover with cold water by 4 inches. Bring to a boil. Reduce heat; simmer, stirring occasionally, until the lentils are tender, about 15 minutes. Drain; return the mixture to the pan. Add 1 teaspoon salt; season with pepper. Stir in 1 tablespoon oil; cover to keep warm.
Cook the bacon in a medium skillet over medium-high heat, stirring occasionally, until crisp. Drain on paper towels. Reserve the fat; wipe the skillet clean. Add the bacon to the lentil mixture.
Melt the butter in a medium saucepan over medium-low heat; add the leek. Cook, stirring, until softened, about 8 minutes. Stir in the cabbage and a pinch of salt. Add 1/ 4 cup water. Cover; cook over medium heat, stirring occasionally, until the cabbage is just tender, 4 to 5 minutes. Season with salt and pepper. Cover to keep warm.
Stir together the mustard and 1 tablespoon oil in a shallow dish. Season the fillets with salt and pepper. Place the fillets in the dish to coat the skins with the mustard mixture.
Add 1 tablespoon reserved bacon fat to the reserved skillet; heat over medium-high heat. In batches, if necessary (allow 1 inch between fillets), cook the fillets, skin sides down, until the skins are browned and very crisp, about 4 minutes. Flip; cook 3 minutes more (for medium-rare).
Add the remaining 2 tablespoons oil to the lentil mixture; stir in the parsley. Divide the lentils and salmon among serving plates. Squeeze the lemon over the cabbage; divide among plates.