Prep the watermelon: Cut the watermelon into 1-inch cubes. Aim for even pieces so the salad looks neat and tosses easily.
Place the cubes in a large mixing bowl.
Slice the cucumber (optional): If using, thinly slice the English cucumber into half-moons. Add to the bowl with the watermelon.
Make the dressing: In a small bowl, whisk together lime zest, lime juice, olive oil, and honey (if using). Add a pinch of salt, a few grinds of pepper, and red pepper flakes if you like heat.
Taste and adjust the lime or honey to your preference.
Toss gently: Pour the dressing over the watermelon and cucumber. Use a large spoon to gently toss, being careful not to break up the fruit.
Add feta and mint: Sprinkle the crumbled feta and torn mint over the top. Give the salad one or two more gentle tosses to distribute without smashing the cheese.
Final seasoning: Taste and add a pinch more salt or lime juice if needed.
Remember, feta is salty, so season lightly.
Serve: Transfer to a serving platter or bowl. For the best flavor, serve immediately while the watermelon is chilled and crisp.