Slice both ends of the watermelon and set on the cut side for stability. Run the knife down the ends, slicing off the rind. Cut watermelon into cubes. Cut the cucumber in half, then into quarters. Chop into pieces. In a bowl, layer the arugula at the bottom, top with watermelon, shallots, and cucumber, then cheese and basil. Sprinkle sumac on top.
Combine the distilled white vinegar, mustard and honey in a small bowl, whisking till combined. Whisk in the olive oil slowly and vigorously till smooth (oil should not be separated).
Drizzle the dressing over the salad when ready to serve, tossing to combine. Top with kosher salt or sea salt and freshly cracked black pepper to taste.
Notes
If using feta cheese, I don't recommend adding salt.
For migraine-friendly, be sure to use sulfite-free dijon and goat cheese or mozzarella, or a young feta.
To make ahead, store the cucumber and watermelon separate from the salad and dressing so it does not get soggy.
This keeps well in the fridge for 2-3 days, but will get soggy if all combined.