To make the vinaigrette, whisk together the lime juice, cumin, lime zest, garlic, mustard, salt and pepper. Whisk in the olive oil. Set aside.
To make the salad, bring a saucepan three-fourths full of water to a boil. Add the shrimp and cook until they curl and turn pink, about 3 minutes. Drain and pat dry. Peel and devein and place in a large non-aluminum bowl. Add half of the vinaigrette to the shrimp, toss well, and let stand for 10 minutes.
Meanwhile, halve, pit and peel the avocados and cut lengthwise into slices 1/2 inch thick. Core and halve the tomatoes and cut into wedges 1/2 inch wide. Add the avocados, tomatoes, and cilantro to the shrimp and toss gently.
Place the salad greens in a separate bowl. Whisk the remaining vinaigrette and pour over the greens. Toss well. Divide the greens among individual plates. Top with the shrimp mixture. Season each serving with several grinds of pepper and serve at once.