How to Make It
In a medium saucepan, cover the lentils with 6 cups of water and bring to a boil over high heat. Reduce the heat to low, cover and simmer, stirring occasionally, until the lentils are tender, about 25 minutes. Season with salt and pepper and set aside, covered.
In a medium skillet, combine the coriander, fennel, cumin and cardamom seeds with the cloves. Toast the spices over moderate heat, stirring, until fragrant, about 3 minutes. Transfer to a plate to cool. Finely grind the spices in a spice grinder or mortar.
In a medium nonreactive saucepan, warm 1/4 cup of the olive oil over moderately low heat. Add the garlic and shallots and cook, stirring, until translucent, about 5 minutes. Add the harissa and 1 tablespoon of the spice mixture and cook, stirring, until fragrant, about 3 minutes. Add the tomatoes and their juices and increase the heat to moderate. Simmer the sauce, stirring occasionally, until the flavors are well blended, about 5 minutes. Season with salt and pepper.
Preheat the oven to 400°. Place 2 large ovenproof skillets over high heat. Add 1 tablespoon of the oil to each skillet. Season the salmon fillets with salt and pepper and coat them on both sides with the remaining spice mixture. When the oil is very hot, add 3 salmon fillets to each pan, skin side down. Put 1 piece of the butter next to each fillet and shake the pans to incorporate the butter into the oil. Sauté the salmon fillets for 3 minutes. Transfer the skillets to the oven without turning the fillets and roast the salmon for about 6 minutes, or until the skin is very crisp and the fish is just cooked through.
Meanwhile, reheat the lentils and the tomato sauce. Spoon the lentils into the center of warmed dinner plates and set the salmon fillets on top, skin side up. Spoon the tomato sauce around the lentils and serve at once.