Heat 1 tbsp of the olive oil over a high heat in a large saucepan. Add the onion and fry for 3 minutes, stirring frequently, until softened. Add the celery, carrot, thyme and vegetable stock. Put the lid on, reduce the heat to medium low and cook for 5 minutes, stirring occasionally. Add the puy lentils, stir to combine and cook for a further 5 minutes with the lid on, stirring occasionally.

Meanwhile, spread 1 tsp of the mustard over the top of the salmon fillets and season lightly with salt and freshly ground black pepper.

Heat a frying pan over a high heat and add the remaining tbsp of olive oil. Cook the salmon skin side down for 3 minutes, then turn them over and cook for 30 seconds to 1 minute, depending on the thickness of the fillets.   Once finished cooking, turn the salmon back onto the skin side. Set the pan aside off the heat for the fish to rest for 5 minutes, while the lentils and vegetables finish cooking.

Stir the remaining 1.5 tsp of mustard, cider vinegar, chopped parsley and tarragon through the lentil and vegetable mixture.