Heat oil and butter in a cast iron skillet (or other heavy oven-proof skillet) over high until melted. Sear tenderloin 5 minutes on one side. Turn and cook another 3 minutes on the other side. Place in the oven and set a timer for 10 minutes.
In another skillet, combine the oil and butter for the sauce over medium-high heat. Ad the onion and cook 5-8 minutes until very soft and slightly browned.
Add the broth, preserves, ginger and mustard and stir to combine. Allow to simmer until the pork is cooked.
Check the pork at 10 minutes. The internal temperature should be 155ºF. Cook up 20 minutes total until internal temperature is 155º.
When the pork is cooked through, remove it from the oven. Place the tenderloin on a cutting board or plate to rest and pour any pan drippings into the apricot sauce.
Add the vinegar to the apricot sauce and remove from heat.
Allow the pork to rest 5-10 minutes before slicing.