Combine the vinegar, mustard, lemon juice, cumin, garlic, oil, salt, and pepper in a jar with a tightly fitting lid. Shake well and set aside.

Soak the onion in cold water for 15 minutes. Drain and combine with the romaine, chickpeas, grapes, cucumbers, chile, and avocado in a large bowl. Add the dressing, salt, and pepper and toss gently. Divide between serving dishes and sprinkle with the queso fresco. Serve.