Heat 3 tablespoons of the olive oil in a medium saucepan over medium-high heat. When it is hot, add 1 cup of the onion and the celery and cook, stirring, until soft, about 4 minutes. Add the lentils, chicken stock, and bay leaf, and bring to a boil. Reduce the heat to a simmer and cook, uncovered and stirring occasionally, until the lentils are just tender, 35 to 45 minutes. Season to taste with salt, and then drain the lentils in a colander. Set aside while you prepare the salmon.

Season the salmon fillets on both sides with salt and pepper to taste. Heat 3 tablespoons of the remaining olive oil in a medium sauté pan over medium-high heat. Place the salmon in the pan, skin side down, and cook until golden brown and crisp, 3 to 4 minutes. Turn the fillets over and cook to the desired degree of doneness, about 2 minutes for medium-rare. Transfer the salmon to paper towels to drain briefly.

Transfer the drained lentils to a mixing bowl and add the remaining 1 /4 cup red onion, the bell pepper, parsley, balsamic vinegar, lemon zest, and the remaining 3 tablespoons olive oil. Toss well to combine, andadjust the seasoning if necessary. Divide the lentils among four wide, shallow bowls, and top each with a salmon fillet. Serve immediately, drizzled with olive oil if desired.