Place oven rack on second level from bottom. Heat oven to 500°. Place loin in a 14-by-12-by-2-inch roasting pan. Rub salt all over loin. Smear 1 tablespoon of the mustard over top of loin. Roast for 20 minutes. Move the loin around with a wooden spatula so it won't stick. Roast 15 minutes more.
Toss the lemon jui ce-coated apples with the oil, then add the apples, juices and all, to the roasting pan, spreading them out around the loin. Roast 10 minutes. Turn apples gently, as they will be soft. Roast 10 minutes more. In total, the loin should roast for 55 min utes, or until meat is done [the USDA says the internal temperature of the meat should be 160°].
The meat may still be slightly pink, but this is fine. Don't overcook. Remove loin to a platter. Snip off strings. If using cream and optional mustard, combine and stir the mixture carefully into apples. Let meat rest 10 to 15 minutes before slicing. Place apples around roast.
NOTES : (6 servings) If adding the optional ingredients, don't stir. Mix the mustard with the cream; pour over the apples, and turn the apples from the bottom in the cream. The sections are soft and fragile.