Trim silver skin from the pork tenderloin and cut crosswise into 12 medallions. Press each medallion with the palm of your hand to flatten to an even thickness. Pat the pork medallions dry with paper towels. Combine 1/4 teaspoon salt, 1/4 teaspoon pepper and the garlic powder, and sprinkle over the pork.
Combine 1 cup of water and the porcini mushrooms in a small microwave safe bowl. Cover the bowl and microwave until steaming, about 1 minute. Set aside to soften; about 5 minutes. Drain the porcini mushrooms (reserving liquid) through a fine-mesh strainer lined with a coffee filter or paper towel. Mince the porcini mushrooms and set aside.
Pour 1 tablespoon of oil into a large skillet set over medium-high heat. Once shimmering, add the pork in a single layer and cook until well browned, about 3 minutes per side. Do this in batches if needed to fit all the pork medallions in one layer. Try not to crowd the pork. Remove to a clean plate and cover to keep warm. Repeat with the remaining pork medallions.
Add the remaining 2 tablespoons of oil to the now empty pan. Once shimmering, add all the mushrooms. Season with 1/4 teaspoon salt and cook undisturbed until well browned. Stir and continue cooking until the moisture is released and then dries up, about 8-10 minutes. Add the shallots and continue cooking, stirring occasionally, until they start to soften, about 3-5 minutes.
Reduce the heat to medium and add the garlic, red pepper flakes and thyme and cook until fragrant, about 1 minute. Stir in flour and cook for 2 minutes, scraping up the browned fond on the bottom of the pan.
Stir in the broth and 1/2 cup of the reserved porcini liquid. Add the Worcestershire sauce and any accumulated pork juices, scraping up any browned bits on the bottom of the pan. Bring to a simmer until the gravy has thickened, about 10 minutes. Check the seasoning and add more black pepper and a little salt, if needed.
Place the pork medallions in the pan nestling them down into the gravy. Cook for 5 minutes or until heated through.
Serve immediately with mashed potatoes and garnished with parsley, if desired.