Using a chef's knife and cutting board, carefully chop the red onion into very small, fine pieces to ensure a balanced flavor distribution in the salsa.
3 cups watermelon chunks
Dice the watermelon into roughly 1-inch cubes and place them into a large mixing bowl. Ensure the pieces are uniform for even texture.
3 cups watermelon chunks
Finely chop the fresh mint leaves and add them to the bowl with watermelon. Use a spoon to gently fold the mint into the cubes, distributing the flavor evenly.
3 cups watermelon chunks
Cut the lime in half and use a lime juicer or reamer to extract the juice directly over the watermelon mixture, aiming for about 1 tablespoon of lime juice. Stir gently to combine.
3 cups watermelon chunks
Carefully chop the chili pepper into very small pieces, and add to the bowl if using. Mix thoroughly to incorporate the chili evenly throughout the salsa. Add salt to taste and stir again.
3 cups watermelon chunks
Let the salsa sit for 5-10 minutes to allow flavors to meld. Serve immediately with a spoon, optionally garnished with additional mint leaves or lime wedges.