Pat the New York strip dry with paper towels. Trim excess fat if desired. Using a sharp knife, make 1/8" slashes in the fat edge at 1/2" intervals to prevent curling. Season both sides lightly with 1/4 tsp kosher salt and 1/4 tsp black pepper.

Slice and prep mushrooms & onion

Slice mushrooms into 1/4"–1/2" thick pieces. Cut the half onion into thin half-moons. Keep them separate and room temperature so they cook quickly on the griddle.

Marinade & seasoning

Make the hibachi-style marinade/seasoning by whisking together 2 tbs soy sauce, 1 tbs mirin, 1 tbs sake, 2 tsp vegetable oil, 1/2 tsp sesame oil, 1/2 tsp sugar, and remaining 1/4 tsp kosher salt and 1/4 tsp black pepper in a small bowl. Reserve about 1 tbs of this mixture for basting and the remainder for marinating or brushing on the steak during cooking.

If you prefer a lightly marinated steak, place the steak in a shallow dish, pour 1 tbs of the mixture over it, turn to coat, and let rest at room temperature for 15–20 minutes. For immediate cooking, simply baste during searing.

Grilling on teppanyaki or skillet

Preheat griddle

Heat a teppanyaki griddle or large heavy skillet (cast iron) over high heat until very hot — about 400–450°F (205–230°C). Lightly oil the surface with 1 tsp vegetable oil and spread with a paper towel using tongs.

Sear the steak

Place the steak on the hot surface and sear without moving for 2–3 minutes to develop a deep brown crust. Flip and sear the other side 2–3 minutes. For a 1–1¼" strip, this yields medium-rare; adjust time ±1–2 minutes per side for desired doneness. During the last minute of cooking, baste the steak with the reserved 1 tbs marinade and 1 tbs butter.

Rest and slice

Transfer the steak to a cutting board, tent loosely with foil, and rest 5–7 minutes. Slice against the grain into 1/4"–1/2" strips for serving.

Cook mushrooms & onions

While the steak rests, add remaining 1 tsp vegetable oil to the hot griddle. Add mushrooms and onions, sprinkle with a pinch of salt (about 1/8 tsp) and pepper, and spread into a single layer. Cook, stirring and flipping occasionally, until mushrooms are browned and onions softened, about 4–6 minutes. In the last minute, add 1 tbs butter and the minced garlic; toss to coat and cook until fragrant (30–45 seconds). Spoon the cooked mushrooms and onions alongside the sliced steak.

Finishing & garnish

Arrange sliced steak and vegetables on a plate. Drizzle any resting juices and remaining browned butter from the griddle over the steak. Sprinkle sliced scallions on top and add a pinch (optional) of shichimi togarashi or chili flakes for heat. Serve with lemon wedges for squeezing over the steak.