01 -  In a small bowl, combine sliced red onion, pickle brine, apple cider vinegar, sugar, and salt. Stir well and let the onions marinate for at least 15 minutes, stirring occasionally. 
 02 -  While the onions marinate, arrange the watermelon cubes in a large serving bowl or platter. 
 03 -  After marinating, drain the onions well (reserve a spoonful or two of the brine if desired for added flavor). 
 04 -  Scatter the pickled onions evenly over the watermelon, then sprinkle crumbled vegan feta cheese over the salad. 
 05 -  Add chopped mint and dill. Season with freshly ground black pepper. 
 06 -  Drizzle a little reserved pickling brine over the salad just before serving for extra tang. 
 07 -  Serve immediately, chilled, and enjoy.