Cut watermelon into 1-inch cubes, removing seeds if necessary. Place in a large bowl.
Thinly slice the red onion. For milder flavor, soak slices in ice water for 10 minutes, then pat dry.
Wash and dry herbs, then roughly tear them to release their aromatic oils.
In a small bowl, whisk together olive oil, lime juice, honey, salt, and pepper until emulsified.
Gently toss the watermelon cubes with the dressing.
Add the sliced onion, crumbled feta, and torn herbs, folding carefully to combine without breaking down the watermelon.
If using nuts, fold them in last.
Chill for 30 minutes before serving to allow flavors to meld (optional but recommended).