Preheat the oven to 375°F. In a medium-sized bowl, whisk together the Dijon and whole-grain mustards, ½ cup of oil, and sherry vinegar until well combined. Set this mustard vinaigrette aside.

Heat a large cast-iron skillet over medium heat. Add 1 tablespoon of the remaining oil. While the oil heats, season the pork chops generously with salt and pepper on both sides. Allow the chops to rest for a few minutes to absorb the seasoning.

Once the oil in the skillet is hot and begins to shimmer, add the shiitake mushrooms, placing them brown-side down in a single layer. Let them cook undisturbed for 3 to 5 minutes until the caps are deeply seared. Using tongs, turn the mushrooms to the other side and continue cooking until golden and crispy. Remove the mushrooms from the skillet and set aside.

Wipe out the skillet with paper towels to remove any excess oil. Add the remaining 2 tablespoons of oil and heat until it begins to smoke. Add the pork chops in a single layer (you may need to cook in batches). Sear the first side for 6 to 8 minutes, or until it develops a rich, brown crust. Flip the chops over, and transfer the skillet to the oven. Roast for an additional 6 to 8 minutes, or until the pork reaches your desired level of doneness. Once done, remove the skillet from the oven and allow the pork chops to rest for 5 to 10 minutes.

Arrange the rested pork chops on a serving platter. Top with the mustard vinaigrette and sautéed mushrooms. Serve immediately and enjoy.