How to Make It
Cut away the peel and pith of the grapefruit. Working over a small bowl, cut along both sides of each grapefruit segment, releasing the segments into the bowl. Squeeze the membrane over a large bowl to release the juice. (You’ll need 3 tablespoons.)
Toss the juice with the onion, ½ teaspoon salt, and ¼ teaspoon pepper in a large bowl. Let sit, tossing occasionally, until the onion has softened, 10 minutes (and up to 1 hour).
Meanwhile, heat 1 tablespoon oil in a large skillet over medium-high heat. Season the steak with the oregano, ½ teaspoon salt, and ¼ teaspoon pepper. Cook until an instant-read thermometer inserted in the thickest part of the steak registers 130° F, 4 minutes per side for medium-rare. Let rest for 5 minutes, then slice against the grain.
Just before serving, add the watercress, grapefruit segments, sesame seeds, wilted onion, and the remaining 2 tablespoons of oil and toss to coat.
Serve the steak with the grapefruit salad, sprinkled with the sesame seeds.