Preheat oven to 425°F (220°C). Place fish on foil-lined baking sheet. Sprinkle with salt and pepper. Cook until fish flakes easily with a fork and is cooked through, about 10 min.
Combine mixed greens, avocado, cucumber, cherry tomatoes and balsamic dressing in a large bowl.
Divide salad among four plates and top each with a piece of salmon.
Use skinless rainbow trout fillets instead of salmon.