Heat a heavy 9 or 10-inch skillet over medium-high flame; add 1 tablespoon of the oil and heat through. Season salmon fillets generously with salt, pepper, and crushed red pepper if using. Add to pan skin side up. Place onions around the salmon and cover. Let cook 4 minutes.
Uncover, turn salmon over and stir onions; add tomatoes. Cover again for 3 minutes or until salmon is firm but still pink and juicy at center. Let rest 1-2 minutes.
Divide the greens and herbs, if using, between two pasta bowls and squeeze half of the lemon over the greens. Sprinkle with salt and pepper; toss. Add a piece of salmon to each bowl at center and squeeze lemon over the fish. Drizzle with remaining tablespoon of olive oil. Serve hot, warm, or at room temperature.