Preheat the oven to 350 degrees F.
In a small pan saute the onions, carrots, celery, and leeks in olive oil until tender. Add the sachet and red lentils to the Sauteed vegetables and stir together. Add the chicken stock, glace de viande, crabmeat, and thyme into the pan, and stir until well mixed. Transfer all ingredients into an ovenproof dish and place in the oven for approximately 1 hour or until lentils are tender.
To make the glaze, combine all of the ingredients in a small pot. Slowly reduce the liquid and aromatics until it becomes a syrup and set aside.
In a small saute pan over medium heat, coat the bottom with the grapeseed oil. Place the snapper skin side down and sear for approximately 2 minutes. Turn fish and place butter and shallots on the skin side. Finish fish under a salamander or broiler basting it with butter.
To serve, place some of the red lentil cassoulet in the center of each plate. Place the snapper in the center and drizzle the glaze around the fish and lentils.