1. 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.
2. 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.