To make Lentil salad, whisk vinegar, oil and mustard in a large salad bowl. Add lentils, tomatoes, cucumber and spinach. Season with pepper and toss well. Cover and refrigerate while cooking salmon.
Place the salmon, skin-side up on a chopping board and pat dry with paper towel. This helps keep the skin crisp (cooking the salmon with the skin on holds it together while cooking and helps it stay moist).
Season salmon with pepper. Heat oil in a large non-stick frying pan over a medium-high heat. Add salmon, skin-side down. Cook for 3 to 4 minutes, or until skin is well browned. Turn salmon and cook other side for 2 minutes, or until cooked to your liking (salmon is best served medium – still a little pink in the centre to retain it’s soft moist texture).
Divide salad and salmon onto serving plates. Serve with lemon.

Salmon fillets and cutlets should have a shiny, moist appearance, with no browning around the edges. Gently press the flesh of the salmon to make sure it springs back slightly.
Check how the fish smells. It should have a pleasant fresh sea smell. Avoid any fish with a strong, unpleasant aroma.