Prepare sliced onions. Place in a small mixing bowl.
Mix lime juice, maple syrup, salt, pepper, and olive oil, if using. Pour over onions and toss. Set aside to marinate for 20 minutes. Toss a few times while marinating.
After 20 minutes, cube the watermelon and place in a large mixing bowl. Add the chopped mint, the feta, and the marinated vegetables. Taste and adjust seasoning. Serve on a bed of arugula.
Note: If you don’t want to make your own feta cheese, we suggest using a good commercial brand. Violife has a wonderful cheese that is very reminiscent of real feta.
This salad is best served the same day but leftovers can be saved in a covered container for one more day.