Oil the grill grates. Grill the onion slices until charred, about 2 minutes per side, then place in a medium bowl.
Place lemon juice, sugar and vinegar into a pot and bring to a boil. Pour the over the onions. Let cool until ready to serve.
Toss the cucumbers, watermelon and pickled onions together and drizzle in some of the brine from the feta and some of the onion liquid to taste. Drizzle in olive oil and season with salt and pepper. Tear the dill and toss in. Add the feta crumbles last and toss one last time.
This recipe has been updated and may differ from what was originally published or broadcast.