Heat oil in a medium skillet pan over a medium-high heat. Add in onion, carrot and walnuts and cook for 3 minutes, until onion softens.
Stir through the lentils for 1 minute, then add in the spinach leaves and stir through until mixture is well-combined.
Remove from the heat and season with a little salt and pepper. Squeeze in half the lemon juice, stir through and serve salad on plate.
Place the pan back over the heat and add in the salmon fillets, skin down. You may need to drizzle a little oil in the pan if it has become a bit dry. Season the salmon with salt and pepper and cook on one side for 4 minutes, or until golden. Flip and repeat on the other side until cooked to your liking.
If your pan has a lid, place it on for the duration of cooking as this will steam the salmon a little quicker.
Plate up the salmon with the salad and remaining wedge of lemon and serve.