Combine the olive oil, lime juice, salt, and pepper in a small bowl and whisk together then set aside. You can also make this in a mason jar and shake it together.
For the Salad:
Combine the cubed watermelon, sliced red onion, and cucumber as well as the torn mint leaves and crumbled feta in a large bowl. Toss together then pour over the dressing and mix to combine.
If you find that raw red onions taste too strong, you can soak them in some ice water for 10 to 20 minutes after you’ve sliced them. This helps take some of the bite out of it as well as make it crisper.
I highly recommend using fresh lime for the dressing. Some bottled lime juices have a slightly bitter aftertaste, and it does not taste as fresh and bright in comparison.
If you’re out of mint, you can use fresh basil instead.
The easiest way to cube watermelon is by cutting the watermelon in half. Then place the halves, cut-side down, and cut a grid pattern into the halves. You’ll end up with long rectangular sticks that you can cube.