2tspblack peppercoarsely crushed with a cast iron pan or in mortar with a pestle
Heat olive oil in a large skillet, add garlic and onions, saute over medium heat until soft and lightly golden brown.
Add spinach and continue sauteing for about 5-7 minutes until the spinach is soft.
Add ground walnuts, 1/2 tsp salt, ground pepper and mix well. Set aside to cool down.
Make a cut in the tenderloin lengthwise, ensuring that you don't cut all the way through - you want to make a pocket. Then make two more cuts holding the knife parallel to the cutting board, along each side of the tenderloin. Again, make sure you don't cut all the way through the meat.
Stuff the spinach and walnut stuffing inside the tenderloin. Close up the meat and tie with a butcher's twine.
Mix the remaining 1 tsp of kosher salt and 2 tsp of crushed black pepper and gently rub the tenderloin with this mix.
Preheat the oven or the grill/smoker to 425F. Cook for about 45-60 minutes or until the desired doneness is achieved. Cover with foil after about 20 minutes to avoid exposed stuffing getting burned.
Check the internal temperature, remove from oven and let sit for 10 minutes. Carefully remove the twine. Slice and serve.