- Heat grill to medium-high. Season the steak with 1/2 teaspoon each salt and pepper and grill to desired doneness, 6 to 8 minutes per side for medium rare. Transfer to a cutting board and let rest.
- Meanwhile, in a large bowl, whisk together the lemon juice, olive oil, honey, and a pinch each of salt and pepper. Fold in the onions and blueberries.
- Cut the watermelon into 1/2-inch-thick triangles and cut off the rinds. Brush lightly with oil, then grill until lightly charred, 1 to 2 minutes per side. Transfer to serving plates.
- Fold the herbs into the tomato mixture, then spoon on top of the watermelon. Slice the steak, divide among the plates and sprinkle with pistachios, if desired.