In a large skillet over medium heat, cook the bacon until it renders its fat and is crisp, about 8 minutes. Transfer the bacon to a paper-towel lined plate, and pour off (and save) all but 2 tablespoons of the fat from the pan.
Season the tenderloins with salt and pepper on one side only and add to the hot skillet, seasoned side down. Season the other side with salt and pepper. Cook until browned on all sides, a total of 8 minutes. Set the tenderloins aside.
Make the gravy: Add 2 tablespoons of the reserved fat to the pan and add the onion, apple, and a generous sprinkling of salt, cooking until the apples are lightly browned, about 5 minutes. Sprinkle the flour over the apples and cook until the flour disappears (is absorbed by the oil). Add the cider, broth, vinegar, and thyme. Bring to a boil, scraping the browned-on bits (fond) off the bottom of the pan to incorporate it into the gravy.
Return the tenderloins to the skillet along with any accumulated juices. Return to a boil, reduce heat to medium-low, cover, and cook until the pork registers 145° F on an instant read thermometer, about 6 to 10 minutes, depending on the thickness of the tenderloin.
Transfer the pork to a cutting board with a well and let rest for 5 minutes. Remove the thyme sprig and taste, seasoning it with salt and more vinegar, as needed, to make it taste bright and sharp.
Slice the tenderloin 1-inch thick and serve with the rich gravy and crumble over the reserved bacon (and don’t fret if your bacon mysteriously disappears before the dish is finished: cook’s treat).
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