Preheat your oven to 400℉. In a small bowl, combine the garlic powder, dried thyme leaves, dried rosemary, dried oregano, salt, and ground black pepper to make a flavorful dry rub. This will ensure a nice, even coating of spices on the pork tenderloin.
Pat the pork tenderloins dry with a paper towel. Rub them all over with 2 tablespoons of olive oil. Evenly coat both tenderloins with the dry rub mixture prepared in Step 1, pressing the seasoning onto the pork to help it adhere on all sides.
Heat a cast iron skillet over medium-high heat and add 1 tablespoon of olive oil. Once the oil is hot, sear the seasoned pork tenderloins on one side for about 1 minute or until golden brown. Turn and sear on the other sides until all sides are golden, which should take just a few minutes. This step is essential for developing a delicious crust.
Transfer the skillet with the seared pork tenderloins into the preheated oven. Roast for about 20 minutes, flipping the pork halfway through the cooking time. Use a meat thermometer to check the internal temperature at the thickest part; it should reach 145℉. If not, continue roasting, checking every few minutes until temperature is reached. Once done, remove from the oven and transfer the pork to a plate. Cover loosely with aluminum foil and let rest for 5 minutes. I like to let the pork rest to keep it juicy and tender.
Place the skillet with the pork drippings back on the stovetop over medium heat. Add the unsalted butter and let it melt, scraping up any flavorful brown bits from the bottom. Add the all-purpose flour and stir well, combining with the drippings and herbs. Slowly pour in the 2% milk, whisking constantly to avoid clumps. Bring the mixture to a boil and let it simmer until the gravy is thickened and smooth, adding more milk if needed to reach your desired consistency. Once thickened, remove from heat.
Slice the rested pork tenderloin and arrange on plates. Drizzle with the rich pan gravy prepared in Step 5 and garnish with chopped fresh parsley for a burst of color and freshness. Enjoy your savory pork tenderloin! For a more authentic flavor, I like to use fresh parsley for garnish rather than dried.