Preheat outdoor grill or grill pan, and brush the grate with the coconut oil.

In a large skillet over medium heat, heat the olive oil. To the skillet, add the next 4 ingredients (garlic through mushrooms).

Cook for 5 to 10 minutes, until vegetables are tender. Stir in spinach and salt and pepper and set aside.

On a large cutting board or plate, place tenderloin. Using a sharp knife, butterfly the beef down the center lengthwise (do not cut all the way through).

Smear the Dijon mustard all over the inside and season well with salt and pepper to taste.

Fill the butterflied tenderloin with the mushroom mixture. Using toothpicks or cooking twine, close the beef down the open seam.

Place the tenderloin on to the grill and cook for 10 minutes per side, rotating until all sides are cooked and crusty and the center is still pink.

Continue to cook until the beef reaches the desired doneness and then remove from the grill and let rest for at least 15 minutes covered in foil.

Slice the tenderloin and serve.