Preheat the oil in a large, cast iron pan over medium-high for 2-3 minutes, a little less if it starts to smoke. Place the slabs of the watermelon onto the skillet, taking care to watch for any sputtering of the water from the watermelon. Turn the heat down just a bit if there is a lot of sputtering. Allow to sear on one side for about 3 minutes, or until it has a nice color, pressing on it slightly with a spatula. Flip and sear the other sides for 2-3 minutes more, or until they have a nice sear all over.
Transfer to a cutting board, and allow to slightly cool. Cut into 1/4-inch thick slices, and transfer to plates. Combine the ponzu and grated ginger, then drizzle evenly over each slab, then sprinkle with a few pinches of smoked salt. Garnish with a few microgreens or scallions if you like and serve with soy sauce.