1. Whisk sour cream, 1 Tbsp lemon juice, and ½ tsp lemon peel in a bowl. Season with salt and pepper. Set aside.
2. Whisk honey, 1 Tbsp olive oil, onion, 2 Tbsp lemon juice, and ½ tsp lemon peel in an 11 x 7 x 2–inch glass baking dish. Add fillets;
turn to coat. Cover; chill up to 1 hour, turning fillets occasionally.
3. Set rack in top third of oven; preheat to 400°F. Line baking sheet with nonstick foil.
4. Transfer salmon to sheet. Roast until opaque in center, about 14 minutes.
5. Toss greens with remaining olive oil, lemon juice, and herbs.
6. Place 1 fillet on each of 6 plates with salad. Serve with a dollop of reserved sauce.