Juice some watermelon. If you start with 1 1/2 cups of fruit you want to reduce the liquid to 1/4 cup. Chill the liquid.
In a bowl, whisk together the mustard with the sherry vinegar. Slowly whisk in the extra-virgin olive oil. Then whisk in the watermelon juice, garlic and shallots. Season the dressing with kosher salt and pepper.
Season the tomato wedges with the sea salt and pepper at least 15 minutes before you toss the salad.
To assemble the salad, put the tomatoes in a bowl. Cut an equal amount of watermelon in blocks and add to the bowl. Add some arugula, red onion, Persian mint, opal basil, feta cheese and some of the dressing. Season with kosher salt and pepper and plate.