Preheat oven to 400 degrees.
In a small bowl, combine your spices (garlic powder, salt, rosemary, black pepper, and thyme).
Pat your loin dry with paper towels, and then press spice mixture into the meat until it is fully coated.
Heat oil in skillet on medium heat. Add in the spice coated loin. Do not touch for 2-3 minutes, to allow a good crust to form. Do a ¼ turn, and allow the next side to brown. Do this with all four sides.
Once the outside of the meat is seared, place in an oven safe dish (we are going to be using the cast iron skillet to make the sauce!) and into a preheated oven and cook until desired doneness is achieved. Around 25 minutes will get you to medium or 150 degrees.
Using the skillet that the pork was cooking in, reduce the heat down to medium-low, and add in butter.
Once the butter has melted, add minced shallots. Saute for 2-3 minutes, until translucent. Add apple cider vinegar and cook for one minute.
Add in heavy cream, chicken stock, and mustard. Whisk to combine. Simmer for 2-3 minutes to reduce.
Allow cooked pork to rest for 10 minutes, and slice. Serve with shallot cream sauce.