In a measuring cup, combine the shallots, sage, apple cider, and cider vinegar. Place the steak in a resealable bag and pour the marinade on top. Close the bag and gently shake to coat the meat. Refrigerate for at least an hour.
Preheat oven to 400 degrees. Prepare a grill or grill pan. Remove steak from marinade, shaking off excess. Season with salt and pepper. Pour marinade into a small saucepan and set aside.
When grill or grill pan is hot, place steak on grill and sear, about 5 to 8 minutes per side. Transfer steak to an oven-proof pan, cover, and bake in the oven another 10 to 15 minutes, or until cooked to desired doneness (cooking time depends on thickness of steak and how rare or well done you like your beef).
While steak is in oven, bring reserved marinade to a boil. Reduce to a simmer and stir in Dijon mustard. Cook about 5 minutes, cover, and keep warm.
When steak is done, transfer to a cutting board and let rest 5 minutes before thinly slicing against the grain.