Take the Tomahawk steaks out of the refrigerator and let them come to room temperature for about 30-45 minutes. Preheat your grill or a cast-iron skillet over medium-high heat.

In a bowl, combine the crushed cashews, chopped rosemary, salt, and black pepper. Rub the Tomahawk steaks with olive oil, then generously coat them with the cashew and rosemary mixture, pressing it into the meat to adhere well.

Place the crusted Tomahawk steaks on the hot grill or skillet. Cook for about 4-5 minutes per side for medium-rare, adjusting the time as needed for your desired doneness. Remove the steaks from the heat and let them rest for at least 10 minutes before slicing.

In a separate pan over medium heat, melt the unsalted butter. Add the sliced leeks and a pinch of salt. Cook the leeks, stirring occasionally, until they are soft and caramelized, about 10-15 minutes.

In a small bowl, combine the Worcestershire sauce and whole black peppercorns. Mix well and set aside.

Slice the rested Tomahawk steaks and arrange them on a platter. Top with the caramelized leeks and drizzle the peppercorn butter over the steak. Garnish with fresh parsley and serve immediately.