Season beef tenderloin with salt and pepper, making sure to cover all sides of the tenderloin.
Heat oil in a large, cast iron skillet until almost smoking. Add seasoned tenderloin and sear on all sides.
Transfer seared tenderloin to slow cooker; add two tablespoons of marsala cooking wine and scrape up browned bits from hot pan. Pour pan drippings over meat. Add beef stock, onion and garlic, and cook on low for 2-4 hours, or until 135 degrees for medium rare. Check at about 1.5 hours for desired doneness.
Remove tenderloin to rest, tented with foil.
Pour meat juices into a saucepan. Add marsala cooking wine, mustard, and starch slurry. Bring to a simmer, then reduce, whisking or stirring constantly, until sauce thickens. Season with salt and pepper to taste.
Slice tenderloin and remove to a platter to serve, topped with pan sauce and fresh herbs.