To a small bowl or mason jar, add the red onion. In a small saucepan, combine the vinegar, water, sugar and salt. Bring the vinegar mixture to a boil then immediately turn off the heat and pour over the onions. Allow to stand at room temperature for at least 2 hours or up to 2 days. Transfer to the refrigerator. Pickled red onions keep in the fridge for 3 to 6 months.

In a serving bowl, whisk together the lime juice, olive oil, pinch of salt and a few turns of freshly cracked pepper. Add the watermelon, sliced jalapeño, cilantro and toss gently until everything is evenly coated in the dressing. Top with shavings or crumbles of queso fresco, pickled red onion and more cilantro.