Cut the ends off the watermelon. Peel the green skin off along the white frontier. Cut 4 slices 2-inches-thick from the peeled watermelon. Trim each slice into a 2-by-4-inch steak.
Cut a small x in the bottom of each tomato. Drop the tomatoes into boiling water for 20 seconds. Drain and submerge the tomatoes in the ice water. Peel and seed the tomatoes. Cut the flesh into 1/2-inch dice. Toss the diced tomato with the vinegar and 2 tablespoons of the olive oil and season with salt.
In a skillet over medium heat, heat the remaining 1 tablespoon of olive oil. Add the watermelon steaks and cook for a few minutes on each side until caramelized around the edges. Place the steaks on plates. Top each with a bit of the tomato mixture, the pistachios and the micro greens. Season with a little Maldon salt and serve.