Heat oven to 400 degrees. Coat a baking sheet lightly with nonstick cooking spray. Season salmon with salt and pepper. Bake 10 to 12 minutes, or until the fish is cooked through and flakes when tested with a fork. Transfer to a plate and let cool.
In a small bowl, whisk together lemon juice and olive oil. Season to taste with salt and pepper. Divide the mesclun greens among 6 martini glasses. Slice salmon into thin strips and arrange on top. Add avocado, cucumber, and tomatoes to glasses. Drizzle with lemon juice/olive oil mixture.