Combine lemon juice, sugar, olive oil, poppy seeds, mustard, garlic, salt, and onion powder in a large bowl. Whisk well until dressing is completely combined.
Add cucumbers, tomatoes, and onion to the bowl. Toss to combine.
Chill until ready to serve. Enjoy!
Video
Notes
If you would like to make this salad ahead, prepare the dressing in a mason jar and toss the salad with it the day you plan to serve it.