Mix the lime juice, oil, salt, pepper and finely diced chilli into a dressing.
Peel the watermelon, remove any seeds and dice the pulp. Halve the avocado, remove the stone, remove the pulp from the skin and also dice. Cut the peeled onion into thin rings. Mix everything immediately with the dressing and let it steep for a moment. Fold in the mint and season to taste.
Arrange the salad on serving plates, sprinkle with crumbled feta cheese and serve garnished with the baby salad.