Heat the oven to 450°F with a rack in the middle position. In a small bowl, stir together the mustard, sugar, coriander seeds, mustard seeds and 1 teaspoon pepper. Measure 3 tablespoons of the mixture into a small bowl and set aside. Rub the remainder all over the pork, then sprinkle with salt.

In a 12-inch oven-safe skillet, heat the oil over medium-high until shimmering. Add the pork and cook, turning occasionally, until browned on all sides, 5 to 8 minutes. Transfer to a large plate.

To the now-empty skillet over medium, add the shallots, apple, vinegar and the reserved mustard mixture. Bring to a simmer and cook, scraping up any browned bits, then add ½ cup water and the dried cherries. Return the pork and any accumulated juices to the skillet. Transfer the skillet to the oven and roast until the center of the thickest piece reaches 135°F or is just slightly pink when cut into, 9 to 12 minutes.

Remove the skillet from the oven (the handle will be hot), transfer the pork to a cutting board and tent with foil. Return the skillet to medium, cover and cook, stirring occasionally, until the apple is softened and the pan is almost dry, about 10 minutes; reduce the heat as needed as moisture cooks off.

Off heat, stir in 2 tablespoons of the tarragon, then taste and season with salt and pepper. Transfer the mostarda to a serving platter, spreading in an even layer. Thinly slice the pork, arrange on top of the mostarda, then sprinkle with the remaining tarragon.