Preheat your oven to 400°F (200°C). Pat the hanger steak dry with paper towels and season generously with salt and pepper.

In a bowl, mix the prepared horseradish, Dijon mustard, and panko breadcrumbs until well combined. Spread the mixture evenly over the top of the steak, pressing it down gently to adhere.

Heat olive oil in an oven-safe skillet over medium-high heat. Once hot, add the steak, horseradish side down, and sear for about 3-4 minutes until browned. Carefully flip the steak to have the horseradish side facing up, then transfer the skillet to the oven. Bake for about 10-12 minutes for medium-rare, or until desired doneness.

While the steak is baking, heat a separate skillet over medium heat. Add the sliced onions and a pinch of salt. Cook, stirring frequently, until the onions are caramelized and golden brown, about 15-20 minutes.

In a shallow bowl, place the flour. In another bowl, pour the buttermilk. Dip the sliced leeks into the buttermilk, then coat them in flour. Heat oil in a pan over medium heat and fry the leeks until crispy and golden brown. Remove and drain on paper towels. Season with salt.

Once the steak is done, remove it from the oven and let it rest for about 5 minutes before slicing against the grain. Serve the steak topped with caramelized onions and crispy leeks, garnished with fresh parsley.