Lemon Vinaigrette: In a small bowl, whisk together all ingredients. Store dressing in the refrigerator until ready to serve. You will need to shake before using each time since the oil and lemon juice will separate. Add salt and pepper to taste.
Prepare Salad: Add mixed greens, smoked salmon, grape seed tomatoes, cucumber, and purple onion into a large bowl and toss well. Drizzle with lemon vinaigrette dressing and serve.