Preheat oven to 325 degrees.
Lay pork loin on a cutting board, and using a sharp knife, make small cuts length-wise, and butterfly; you want to end up with a rectangle of pork approximately 1” thick. Season both sides with salt and pepper and set aside.
In a saute pan over medium heat, melt butter. Add onions, apples, and sage, and cook for 3 minutes until just softened; season with salt and pepper.
Pour mixture into a bowl with bread and mix to combine evenly.
Spread apple/onion mix evenly across pork, and gently roll back into a cylinder; tying with kitchen string as needed; place in baking pan.
In a small bowl whisk together cider, mustard, and oil until combined; smear all over exterior of pork loin.
Place in oven and cook until center reads at 145 degrees with a thermometer; remove and let stand for 10 minutes before carving.
Apples and pork are a natural combination, and this dish has great earthy flavors without being too heavy for a summer get-together.
Sage is a hearty enough herb to stand up to most flavors, and pairs very well with pork in this dish.