Place sliced onion 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 or more. Toss a few times while marinating.
Cube the watermelon and place in a large mixing bowl. Add the chopped mint, the feta, onions, and blueberries. Taste and adjust seasoning. Serve on a bed of baby arugula.
Note: If you want to make your own vegan feta cheese, try our recipe here. https://veggiefestchicago.org/recipe/mediterranean-herbed-feta-from-the-non-dairy-evolution-cookbook-by-chef-skye-the-gentle-chef/