Pre-heat oven to 425 degrees. Add vegetable oil to a cast-iron skillet. Season pork loin with salt and pepper. Add pork to the skillet, browning on all sides, 2minutes per side, about 8 minutes total.
Place skillet with both tenderloins into oven for 10-15 minutes, or until the internal temperature reaches 145-150 degrees.
Remove from oven, set the tenderloins aside on a plate while you make the pan sauce. Using the skillet, add green onion, cook for one minute. Add the cognac, bring to a boil, deglazing the pan by scraping the pork bits off of the bottom of the pan. Add stock, bring to a simmer and let reduce for 8 minutes or so, until the sauce is reduced by a third.
Turn the heat to medium and stir in the heavy cream, cook for another 3 minutes, until the sauce thickens.