Combine the diced tomatoes and cucumbers in a large bowl.
In a blender or food processor, combine the olive oil, vinegar, sugar, yogurt, mustard powder, salt, and garlic powder. Blend/process for 15 to 20 seconds. Add the poppyseeds and pulse a few times until incorporated.
Pour dressing over salad, to taste (you probably won't use it all...refrigerate any leftover dressing). Either serve salad immediately or chill for several hours and then stir before serving.
Notes
Large tomatoes should be diced, but grape or cherry tomatoes may be halved or left whole, depending on size. You may seed the cucumbers if they are large and contain a lot of seeds.
You may use plain regular yogurt if you don't have Greek.