Heat the oven to 400 degrees. Toss the root vegetables with 2 tablespoons olive oil, half the salt, and half the pepper in a large roasting pan.
Mix the remaining olive oil with the garlic, rosemary, thyme, parsley, and the rest of the salt and pepper to form a paste.
Pat the pork tenderloin dry and rub the herb paste evenly over the surface.
Nestle the pork into the center of the vegetables in the roasting pan.
Roast until the pork reaches an internal temperature of 145 degrees and the vegetables are tender and browned at the edges.
Remove the pork to a cutting board and let it rest while you make the gravy.
Place the roasting pan on the stovetop over medium heat or pour the pan juices into a saucepan. Add the butter and flour, stirring to form a roux.
Slowly whisk in the apple cider and chicken broth, scraping up any browned bits. Simmer until the gravy thickens to a light coating consistency.
Slice the rested pork and serve it with the roasted vegetables and warm gravy.