Marinate Pork Tenderloin: First trim the pork of any tough skin or very thick fat if there is any. Stab pork tenderloin all over with a fork (like 10-12 times). Salt both sides of the pork. Dust dry spices (paprika, onion powder, garlic powder and black pepper) evenly over the pork tenderloin and rub it in. In a large bowl, whisk together A1 steak sauce & soy sauce. Reserve 1/4 cup to add before baking. Add pork tenderloin to marinade & coat all over with marinade. Cover the top of the bowl & let marinate at least 30 minutes or up to 8 hours.
Cook Pork: Preheat oven to 375° when you are ready to cook the pork. In a large, oven safe skillet or cast iron heat avocado oil on high heat until just smoking. You can also use a large skillet and transfer to baking dish. Add pork tenderloin and cook 5-6 minutes on one side on high heat until a light crust has formed & it is golden.If using a cast iron, flip the pork & remove it from the heat. Pour reserved 1/4 cup A1 marinade over the tenderloin & place it in the oven. If using baking dish: Brown the pork for 5 minutes in large skillet. Remove from heat and place pork in a baking dish. Pour any pork juice from pan over the top as well as reserved 1/4 cup marinade.
Bake 25-30 minutes or until thickest part of reads 145°. Let rest 10 minutes before slicing & serving.