Chop the onions brunoise and transfer it to a bowl with 1/2 tablespoon of vinegar, ice cubes and cover with water. Keep the onions immersed for at least 10 minutes to lose the acidity.
Dry the onions and return to a bowl. Add the watermelon in cubes, the lemon juice and zest, the remaining vinegar, and season with salt. Mix it gently to not break the watermelon cubes and keep in the fridge until serve.