How to Make It
In a large, shallow baking dish, combine the soy sauce, vinegar and sesame oil. Add the steak, turn to coat and let stand at room temperature for 2 hours or refrigerate for up to 4 hours; turn often.
Meanwhile, in a small skillet, toast the cayenne pepper over moderate heat until fragrant, about 10 seconds. Scrape the cayenne into a small bowl and add the vegetable oil and a pinch of salt. Let stand at room temperature for at least 2 hours.
In a bowl, combine the diced radishes, zucchini and fresh cayenne chile. Cover and refrigerate for up to 4 hours.
Preheat the oven to 400°. In a large ovenproof skillet, heat the olive oil until almost smoking. Remove the steak from the marinade, season with salt and sear over moderately high heat until deeply browned on the bottom, about 4 minutes. Turn the steak over and roast in the oven for about 10 minutes, or until an instant-read thermometer inserted in the center of the meat registers 120°. Transfer the steak to a carving board and let rest for 10 minutes.
To serve, slice the steak against the grain 1/4 inch thick and arrange on 4 plates. Garnish with the radish relish, drizzle with the cayenne oil and serve.