Combine 1/4 cup water, 2 tablespoons vinegar, and 1 tablespoon agave in a small bowl. Add onion; toss to coat. Marinate at room temperature for 30 minutes; drain.
Combine remaining 2 tablespoons vinegar, remaining 1 teaspoon agave, mustard, pepper, and salt in a large bowl, stirring with a whisk. Gradually add oil, stirring constantly with a whisk. Add salad greens; toss gently to coat. Arrange about 1 cup salad on each of 8 plates. Top each serving with about 1 tablespoon onion, 1 tablespoon cheese, and about 6 raspberries.
MyRecipes is working with Let's Move!, the Partnership for a Healthier America, and USDA's MyPlate to give anyone looking for healthier options access to a trove of recipes that will help them create healthy, tasty plates. For more information about creating a healthy plate, visit www.choosemyplate.gov.