1. Heat a grill or grill pan over high heat until very hot. Two at a time, grill watermelon rounds, turning once, until charred (about 2-3 minutes per side). Transfer watermelon to a cutting board and remove rind; cut rounds into wedges.
2. Arrange watermelon on a serving platter; sprinkle with mint and feta. Drizzle with olive oil and vinegar, then sprinkle with salt and pepper.