For each Tenderloin (which serves about 3 people), assemble the ingredients near your stove.
Put the tenderloin in the oven and set the timer for 7 minutes.
In a large skillet (cast iron if you have it) heat 2T. Olive oil and 2 T. Butter over medium-high heat until bubbly.
Add onions, salt and pepper, sugar and thyme sprigs. The sugar helps the onions caramelize and you'll pull the thyme stems out later after the leaves have fallen off.
Sauté over med-high heat for 3 or 4 minutes, move to the perimeter of the pan, and add the apples and sage. Leave them undisturbed to brown for a couple of minutes.
When the timer goes off, pull the oven rack out, flip the tenderloin over, and put it back in. Set the timer for 6 minutes. Internal temp should be 140. Remove, cover with foil, and let rest for 5-7 minutes while you finish the apples.
Stir the apples. Sauté for another 2 minutes or until they are just getting soft but not mushy, then add apple cider, mustard, and vinegar. Make sure the mixture is bubbling hard and as soon as it coats the back of a spoon, take it off the heat and cover.
If you have 5 or 7 minutes to wait before you can slice the pork, we suggest braised cabbage or polenta as an additional side dish.
Place the apples and onions as a bed on a platter, slice the tenderloin and fan it out in a straight line on top, and garnish with thyme and sage.