Make the dressing: place yogurt, 1/2 cup cilantro, jalapeno, garlic, lime juice and salt in a blender. Cover and blend until smooth. Set aside.
Combine: tomatoes, cucumber, avocado and onion in a large bowl. Add dressing. Gently toss all together until combined. Cover and chill for 30 minutes before serving.
Garnish with chopped cilantro and some pepper, if desired.