In a saucepan over medium-high heat, combine the lentils and 3¼ cups (26 fl oz/820 ml) of the stock and bring to a boil. Reduce the heat to medium-low, cover, and simmer until the lentils are tender but not mushy, about 30 minutes. (You may need to add a bit of water toward the end.) Drain and set aside.

In a frying pan over medium heat, heat 3 Tbsp of the oil. Add the onion and sauté until softened. 5-7 minutes. Add the celery and carrot and sauté until slightly softened, about 2 minutes. Add the bell pepper and sauté until softened, about 2 minutes.

Add the lentils to the pan and sauté over medium heat to allow the flavors to blend, about 2 minutes. Stir in the lemon juice, the remaining l Tbsp oil, the chopped parsley and basil, and season to taste with salt and pepper. Transfer I cup (7 oz/220 g) of the lentil mixture to a blender or food processor. Process to a puree, adding just enough of the remaining¼ cup (6 fl oz/180 ml) stock to produce a sauce-like consistency. Return to the pan with the whole lentils and mix well. Taste the puree and adjust the seasoning, then cover, set aside, and keep warm.

Preheat the oven to 450ºF (230ºC). Season the salmon with salt and pepper. Place the fillets on a baking sheet and roast just until opaque throughout, about 12 minutes, depending on the thickness of the fillets.

Mound an equal amount of the lentils on individual plates and place a salmon fillet on top of each mound. Garnish with the parsley sprigs and basil leaves and serve at once.