Preheat the grill to 500F (260C).
Remove the rind from the watermelon, and cut the flesh into thick strips that are roughly 1.5 inches thick and 6 inches long.
Remove the stem section from the tops of the beefsteak tomatoes, and cut the tomato flesh into large ¾-inch thick rounds.
Salt the tomatoes lightly with Kosher salt on both sides.
Gently grill the watermelon and tomatoes at 500F (260C) until they have some grill marks on the outsides.
Set the watermelon and tomatoes aside on a plate for whenever you are ready to assemble the salads.
Combine the sliced sweet onion with the vinegar, honey, and Dijon mustard. Mix to combine.
Cut the grilled watermelon and tomato into bite-sized pieces, and divide the pieces among 4 salad bowls.
Add the onions with the dressing on top of the watermelon and tomato.
When you are ready to serve, top each individual salad with some toasted pine nuts, Feta cheese, sliced fresh mint, extra virgin olive oil, and freshly-ground black pepper.