Cover a large platter with the spring mix greens. Layer on grapefruit sections, jicama, avocado and red onion. Drizzle with the poppy seed dressing.
Poppy Seed Dressing: Combine sugar, vinegar, salt, mustard and onion in bowl and whisk until the sugar is dissolved. Slowly add in the oil in steady stream and whisk until it has emulsified. Stir in poppy seeds.