Using a round shaped cookie or biscuit cutter, make round circles of watermelon. Alternatively, you can cut the watermelon into squares or triangles. Place the watermelon pieces into a plate or serving platter and sprinkle with some Tajín. Use as little or as much as you prefer.
Add the feta cheese slice on top of half of your watermelon rounds. If your feta crumbles, that's fine. Just top each watermelon piece with feta. Top with the another watermelon round to make a sandwich. Sprinkle more Tajín and/or some balsamic glaze. Garnish with fresh mint or basil if wanting to add something green. Keep cold, serve and enjoy!