Preheat the oven to 325°F.
Tie the pork loin at 2-inch intervals with kitchen twine to hold its shape.
Combine the flour, cayenne, salt, pepper, and rosemary in a small bowl.
Rub this mixture evenly all over the loin.
Heat 2 tablespoons of the butter in a large heavy skillet or roasting pan and sear the meat over high heat, turning often until evenly browned.
Transfer the loin, with the pan juices, to a large baking pan. 

Scatter the onions and garlic around the roast.
Cut up the remaining butter and distribute evenly over the vegetables.
Cover with foil and place in the oven.
Cook for 45 minutes, then add the apples and the cider.
Baste everything with the pan juices.
Cover and cook for 30 minutes more.
Raise the oven temperature to 400°F and remove the foil.
Baste and cook for another 15 minutes.
Remove from the oven and transfer the loin to a cutting board. Carefully remove the twine and let stand for 10 minutes. 

Meanwhile, transfer the onions and apples to a platter.
On top of the stove, reduce the pan juices by half.
Warm the Calvados and carefullypour into the pan.
It should flame, then die down. Keep a pan lid nearby in case the Calvados flares up.
Simmer the sauce while you slice the pork loin. Arrange the meat over the apples and onions and serve with the sauce.