Pre-heat the oven to 425. Butterfly the steak, look for it to be between ¼”-1/2” thick. Place it on a cutting board.
If you spot parts that are unevenly thick, flatten them out with a meat mallet. Season the steak with salt and pepper and set aside.
Place a medium-large skillet over medium heat. Add 1 tablespoon of olive oil, once the oil is hot add the chopped onion and minced garlic. Cook for 3-4 minutes, stirring occasionally, until the onion is soft and starts to turn golden brown at the edges.
Add chopped spinach to the skillet and cook for 3-4 minutes, stirring occasionally, until the spinach is soft. Set aside to cool off.
In a large mixing bowl, combine ground walnuts, crumbled blue cheese and the spinach mixture. Stir well to combine. Season with salt and pepper to taste.
Turn the steak so the “grain” of the steak is running left to right in front of you. Spread the spinach mixture evenly across the surface of the butterflied steak, one inch away from the sides of the steak.
Starting with the side closest to you, roll the steak tightly forward, around the filling, until the roll is complete. Use twine to tie up the rolled steak about every 2 inches. If you see any filling escaping out the sides, tuck it back in.
Add 1 tablespoon of olive oil to a 9×13 baking dish, brush the sides and bottom of the dish with the oil.
Transfer the stuffed steak roll to the baking dish and using a pastry brush, brush the remaining 1 tablespoon of olive oil all over the surface of the steak.
Sprinkle with the remaining 1 tablespoon of black pepper. Place in the pre-heated oven and bake for 35 minutes. Turn the oven to broil and broil for 5-10 minutes. Watch it closely not to burn it.
Remove from the oven, cover the steak with aluminum foil and allow the steak to rest for 15 minutes before removing the twine and slicing.
Cut into slices with a serrated knife and serve.