For the dressing: In the bowl of a food processor, combine the yogurt, oil, scallions, garlic, cilantro, lime zest and juice and cumin and season with salt and pepper. Blend until smooth.
For the salad: In a bowl, toss together the tomatoes, onion and cilantro and season with salt and pepper; set aside.
Cut the avocados in half, remove the pits, and use a spoon to scoop out the flesh from each half. Slice off a small amount from the bottom of each half so that they sit flat on your platter. Squeeze a little lime juice on the cut side of each avocado and season lightly with some salt. Scoop about 1/4 cup of the tomato mixture inside each half (it's good if a little extra falls around the avocado) and drizzle with some of the dressing. Serve right away!