For the lentil stew:
In a large, deep sauté pan, melt the butter with the olive oil over medium heat. Add the onion, leeks, celery, and carrots. Sauté the vegetables until they are tender, and the onion and leeks are translucent, about 15 to 20 minutes.
Add the garlic, 1-½ teaspoons of the kosher salt and ¼ teaspoon of the pepper and cook for another 3 minutes.
Stir in the chicken base and tomato paste along with 6 cups of the chicken stock, thyme sprigs, and bay leaves. Add the lentils.
Bring to the stew to a boil then reduce the heat to medium-low. Cover the pan and cook for about 45 minutes or until the lentils are very tender, stirring occasionally and adding more chicken stock as needed. When the lentils are tender, stir in the red wine vinegar and check for seasoning, adding additional kosher salt and pepper if needed. Keep the stew warm over low heat while you cook the salmon.
For the salmon:
Preheat the oven to 400-degrees. Brush both sides of the salmon with olive oil. Sprinkle the flesh side with kosher salt and ¼ teaspoon pepper. Let the salmon rest at room temperature for about 10 to 15 minutes to take the chill off.
Spray a large ovenproof skillet with a cooking spray. Place the pan over heat slightly below medium-high. As soon as the pan is hot, place the salmon fillets flesh side down. Leave them in place without moving for 3 minutes.
Flip the fillets over and place in a 400-degree oven for 5 to 7 minutes or until the fish is cooked through and flaky. The cooking time will vary depending on the thickness of the salmon. Do not overcook or the salmon will be dry.
To serve, ladle the lentil stew into a wide soup bowl and gently place the salmon fillet on top.
To reheat any leftovers:
Warm the lentil stew over medium heat in a covered saucepan, adding extra chicken stock if needed. As soon as the stew is hot nestle the remaining salmon on top of the stew. Cover and warm for an additional 10 minutes until the fish is warmed through.