Whisk together first 8 ingredients in a small bowl.
Combine romaine lettuce, carrots, grapefruit segments, and sliced onion in a large bowl. Sprinkle with sunflower seeds, if desired. Serve with vinaigrette.
Note: Store remaining vinaigrette in an airtight container in refrigerator up to 1 week.
Note: Nutritional analysis without seeds: Calories 104 (45% from fat); Fat 5.3g (sat 0.6g, mono 2.1g, poly 2.2g); Protein 1.3g; Carb 15.4g; Fiber 2.2g; Chol 0mg; Iron 0.7mg; Sodium 209mg; Calc 35mg.
Note: Nutritional analysis below includes sunflower seeds.