If using pork loin, slice it 1/4 inch thick. Pound the slices or cutlets between two sheets of waxed paper, using the side of large cleaver or the bottom of a small pan, and using a light slapping motion. Pound them to 1/8 inch thick. In a large skillet, heat 2 tablespoons of the butter over medium-high heat. When the bubbling subsides, add the onion and apple and cook, stirring occasionally, until golden and tender, about 10 minutes. Meanwhile, spread the pork paillards with a thin layer of mustard and sprinkle with 1/2 teaspoon salt. When the onion and apple are ready, stir in the marjoram. Transfer the mixture to a plate, wiping out the pan thoroughly, and cover the mixture to keep it warm. Turn the heat up to high and add the oil to the pan. Cook the pork in batches for 1 minute on the first side, turn, and cook until lightly browned, about 1 minute. Transfer to a serving platter and keep warm.

Pour any juices from the meat platter into the pan, and add the vinegar. Deglaze -- don't reduce -- over medium heat, stirring and scraping up the browned bits with a wooden spoon. Return the apple-onion mixture to the pan; heat through. Swirl in 1 tablespoon butter, if you like, season with salt and pepper, spoon over the meat, and serve.