To make this watermelon feta salad, start by making the candied walnuts. Warm a nonstick skillet over medium heat and add the butter. Once it's melted, add the walnuts and coconut sugar and stir to coat everything well.
Continue cooking until the sugar has fully dissolved, then remove the pan from the heat and transfer the walnuts to a tray. Pop the tray in the freezer for 20 minutes to let them harden, then break the clusters apart once they're set.
While the walnuts are in the freezer, prepare the salad. In a very large bowl, combine the cubed watermelon, chopped strawberries, cucumbers, feta, red onion, and mint.
Next, make the dressing. In a small bowl, whisk together the olive oil, honey, lime zest and juice, tajin (if using), and kosher salt until fully combined.
Pour the dressing over the salad and toss gently to coat. Add in the candied walnuts right before serving so they stay crunchy, then enjoy immediately.