In a small bowl, combine red onion, apple cider vinegar, sugar, and salt. Let pickle for at least 15 minutes while preparing other components.
In a mixing bowl, whip ricotta, heavy cream, and honey until light and fluffy. Season with a pinch of salt.
Heat grill to medium-high. Brush watermelon slices 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 for 2-3 layers.
Drizzle with pickled onion vinaigrette (including some pickled onions) and garnish with fresh mint leaves.