Heat the oven to 425°F (220°C). Take the tenderloins out of the fridge 20 minutes before cooking so they roast evenly, and pat them completely dry with paper towels — a dry surface is what lets the crust stick and brown.
Make the herb crust: stir the rosemary, thyme, garlic, lemon zest, salt, pepper, cinnamon, 2 tablespoons olive oil and the Dijon together into a thick, fragrant paste. Rub it over every side of both tenderloins, pressing it on with your hands so it adheres in an even layer.
Sear for color and flavor: heat the remaining 1 tablespoon of olive oil in a large oven-safe skillet over medium-high heat until it shimmers. Lay the tenderloins in and sear for 2 minutes per side, 8 minutes in total, until the crust is deep golden in spots. Do not skip this — the sear is where the crust gets its flavor.
Transfer the skillet to the oven and roast at 425°F (220°C) for 15 to 18 minutes, until an instant-read thermometer pushed into the thickest part reads 145°F (63°C). Tenderloins are lean and cook fast; start checking at 15 minutes, because 5 extra minutes is the difference between juicy and dry.
Move the pork to a cutting board, tent it loosely with foil, and rest it for 10 minutes. The temperature will hold and the juices will settle back into the meat — slicing early sends all of that juice onto the board instead.
While the pork rests, make the glaze in the same skillet: pour in the apple cider, honey and apple cider vinegar, scraping up all the browned bits from the bottom. Simmer over medium heat for 6 to 8 minutes, until it has reduced by about half and coats the back of a spoon in a glossy layer. Take it off the heat and whisk in the butter and the pinch of salt until it shines.
Slice the tenderloins into 1/2-inch medallions — you should see a blush of pale pink in the centers, which is exactly right at 145°F (63°C). Fan the slices out on a warm platter, spoon the warm glaze generously over the top, and serve the rest of the glaze at the table.
