Remove silver skin from tenderloins. Cut a slit all the way down the long end of your tenderloin, making sure not to cut all the way through. Open the tenderloins like a book, cover with a sheet of plastic wrap & pound with a meat mallet until they are about 1/2-inch thickness.
Divide filling mixture between the two tenderloins & spread evenly over the surface of the tenderloins, leaving 1/2-inch at the borders. Roll tightly starting with the long end & secure the ends with toothpicks. Season all over with salt & pepper.
Preheat oven to 400 F. Heat a large oven proof skillet with 2 Tbsp oil. Once oil is hot, place tenderloins in the skillet & sear about 2 minutes per side. Transfer the skillet with the tenderloins to the oven & bake for about 18-20 minutes or until thermometer reads 145-150 F. in the thickest portion of the meat. Transfer to a cutting board, brush with pan drippings. Cover loosely with foil & allow to rest 10 minutes before slicing.
Cabbage & Apples
While tenderloin is roasting, prepare cabbage/apple mixture.
In a large skillet over medium heat, cook onion in butter until soft & translucent, 3-4 minutes. Add garlic & continue cooking just until fragrant, 1 minute more.
Add the cabbage & continue cooking until wilted, 6-8 minutes. Season to taste with salt & pepper. Continue cooking until cabbage begins to caramelize, 4-5 minutes longer.
Add the cubed apple, cider, mustard & brown sugar; carefully combine. Cover & cook until any liquid has evaporated & apples are soft. Place on a serving platter. Top with sliced tenderloin medallions & serve. If you wish, you could also serve the tenderloin with some mashed potatoes & oven roasted carrots.