Preheat oven to 400 F. Generously season the pork tenderloins with salt and pepper.
Heat the vegetable oil in a large skillet over high heat. Place the seasoned tenderloins in the skillet and brown on all sides.
Transfer tenderloins to a sheet pan.
Bake the tenderloins for 18 - 25 minutes, or until a meat thermometer inserted into the thickest part reads 145 F. Cover and let rest 10 minutes before slicing.
As the tenderloins bake, reduce the heat of the skillet to a medium-high, and add the onion and garlic. Saute for 1 minute before de-glazing the pan with chicken stock. Scraping any browned bits from the bottom of the pan.
Add the thyme, mustard, and onion powder, and let simmer until liquid has reduced by half.
Remove from heat, stir in heavy cream, and season the sauce to taste with salt and pepper. Serve this sliced tenderloin drizzled with the mustard sauce.