In a small bowl, whisk together the maple syrup, mustard, rosemary, lemon juice, and soy sauce. Pour over the pork and marinate for at least 2 hours, or overnight.
When you're ready for dinner, preheat the oven to 400F. Heat the oil in a large skillet set over medium-high heat. Remove the pork from the marinade, reserving the marinade, and brown the tenderloins on all sides, turning as necessary. This should take about 5 minutes.
Transfer the pork to a baking dish and bake for 15-20 minutes. (If you have a meat thermometer, it should register 155°F/68°C). Transfer the pork to a cutting board, cover it with foil, and let it stand until you’re ready for it.
Meanwhile, add the butter to the skillet (don’t wash it out!) and sauté the apples for 5-7 minutes, until the apples are tender and golden. Transfer the apples to a plate. Add the marinade and apple cider to the pan and bring to a simmer, scraping up any flavourful browned bits stuck to the bottom of the pan.
Pour a small amount of the sauce (about 1/4 cup) into a small dish, whisk in the cornstarch until you get rid of all the lumps (this is called a slurry) and return the mixture to the pan. Simmer for about 5 minutes, until the sauce is slightly thickened. Return the apples to the sauce along with any juices that have collected on the plate.
Slice the pork and serve it topped with the apples and sauce.