Place cast-iron skillet in oven, preheat to 400 degrees.
Dry tenderloin with a paper towel, set aside. Combine mustard, rosemary, garlic, salt and pepper. Coat tenderloin with mustard mixture, then dust all sides with almond meal.
Remove cast-iron skillet from oven and transfer to stovetop. On a medium heat, sear each side of the tenderloin for approximately 2 minutes (keep close watch, to assure it does not burn). After all sides have been seared, sprinkle rosemary and chopped almonds over all sides of the tenderloin. Transfer to oven and roast for 8-10 minutes, until internal temperature reaches 115 degrees in the thickest area of the meat.
Remove tenderloin from the oven and let rest for 10-15 minutes before carving and serving.