
1. Place the salmon fillets in a shallow bowl. Toss well with lemon juice, olive oil, salt and pepper. Let rest for 15 minutes.
2. Cook the salmon, skin-side down in a nonstick skillet over medium-high heat for 2 to 3 minutes, shaking the pan and carefully lifting the salmon with a spatula to loosen it from the pan.
3. Reduce the heat to medium. Cover the pan and cook until the salmon is cooked through, 3 to 4 minutes more. The skin should be crisp and the flesh medium-rare.
4. Meanwhile, combine the arugula, tomatoes and onion in a bowl. Just before serving, season with salt and pepper and drizzle with oil and vinegar. Toss well.