In a small bowl, combine red onion, apple cider vinegar, sugar, and red pepper flakes. Let pickle for at least 30 minutes.
In a separate bowl, whip ricotta with honey and sea salt until light and fluffy. Refrigerate until ready to use.
Heat grill to medium-high. Brush watermelon rounds with olive oil and grill for 2-3 minutes per side until grill marks appear.
To assemble, place one grilled watermelon round on a plate, top with a dollop of whipped ricotta, then another watermelon round. Repeat to create a tower.
Drizzle with pickled onion vinaigrette and garnish with fresh mint leaves.