Peel (optional) and cut potatoes into 1/3"–1/2" sticks. Rinse in cold water until water runs clear to remove surface starch. Soak in cold water for at least 30 minutes (up to 2 hours) to remove more starch and improve crispness.

Drain the potatoes and dry thoroughly with a kitchen towel or paper towels. Toss with the optional cornmeal or potato starch for extra crunch (use about 1 tsp) and a light dusting so they are dry but coated.

First fry (blanch)

Heat oil in a heavy pot or deep fryer to 300°F (150°C). Fry potatoes in batches for 4–6 minutes until tender but not colored. Remove and drain on a wire rack or paper towels. Let cool while you cook the steak (can be held at room temperature briefly).

Final fry

When ready to serve and after steaks are rested, heat oil to 375°F (190°C). Fry the blanched fries in batches until golden brown and crisp, 2–3 minutes. Drain on paper towels, immediately season with 1 tsp kosher salt and 1/4 tsp black pepper. Keep warm in a single layer in a low oven (200°F/95°C) if needed.

Steak & seasoning

Bring the Wagyu hanger steak to room temperature for 30–45 minutes. Pat very dry with paper towels — dryness is essential for a good sear.

Season both sides of the steak generously with 1.5 tsp kosher salt. Press the coarsely cracked black pepper into both sides so it adheres well.

Sear the steak

Heat a heavy skillet (cast iron recommended) over high heat until smoking. Add 1 tbsp neutral oil and swirl. Add the steak and sear without moving for 2–3 minutes, until a deep brown crust forms. Flip and add 1 tbsp unsalted butter. Continue cooking and spooning melted butter over the steak for another 2–3 minutes for medium-rare (internal temp 125–130°F/52–54°C). Adjust time if you prefer rarer or more done. Remove steak to a cutting board and tent loosely with foil to rest for 8–10 minutes; reserve pan fond for sauce.

Green peppercorn sauce

After removing the steak, reduce heat to medium and add 1 tsp vegetable oil to the hot pan if dry. Add the minced shallot and sauté 1–2 minutes until softened and translucent, stirring to dissolve browned bits.

Deglaze the pan with 1/4 cup dry white wine or cognac, scraping up browned bits. Allow the alcohol to evaporate and reduce until almost evaporated, about 1–2 minutes.

Add 2 tbsp drained green peppercorns and 3/4 cup demi-glace (or concentrated beef stock thinned slightly to similar volume). Stir and simmer gently 2–3 minutes to combine and reduce slightly.

Stir in 1 tsp Dijon mustard and 1/2 cup heavy cream. Simmer over medium-low heat until sauce thickens enough to coat the back of a spoon, about 3–4 minutes. Taste and season with 1/4 tsp salt if needed.

Turn off the heat and whisk in 1 tbsp cold cubed unsalted butter to finish the sauce and give it a glossy sheen. Adjust consistency with a splash of water if too thick. Keep warm until plating.

Plating & finishing

Slice the rested Wagyu hanger steak against the grain into 1/2"–3/4" thick slices.

Arrange slices on warmed plates. Spoon the green peppercorn sauce over the steak or serve alongside. Add a portion of hot frites to each plate.

Garnish with a light sprinkle of chopped parsley and a lemon wedge on the side if desired. Serve immediately.