In a medium bowl, combine the softened butter, sun-dried tomatoes, basil, garlic, salt, and pepper. Mix until well combined. Place the mixture onto a piece of plastic wrap and roll it into a log shape. Twist the ends to seal and refrigerate until firm, about 30 minutes.

Preheat your oven to 400°F (200°C). Season the hanger steak generously with salt and pepper on both sides. In a shallow dish, combine the crushed walnuts and rosemary. Press the seasoned hanger steak into the walnut and rosemary mixture, ensuring it is well coated on all sides.

Heat the olive oil in a large oven-safe skillet over medium-high heat. Add the coated hanger steak to the skillet and sear for about 3-4 minutes on each side, until browned. Transfer the skillet to the preheated oven and roast for another 5-7 minutes for medium-rare, or until your desired doneness is reached. Remove the skillet from the oven and let the steak rest for 5 minutes before slicing.

Slice the hanger steak against the grain and serve with slices of the sun-dried tomato compound butter on top. Enjoy with your favorite sides!