Heat the oven. Arrange a rack in the middle of the oven and heat the oven to 400°F.

Prep the ingredients. Quarter and core 2 tart-sweet medium apples. Peel and cut 1 large yellow onion into eighths. Cut 2 tablespoons unsalted butter into ½-inch pieces.

Make a garlic paste. Coarsely chop 4 garlic cloves. Sprinkle with 2 teaspoons kosher salt. Hold the blunt side of the knife with both hands, angle the knife slightly, and scrape the pile of garlic and salt to flatten. Use the knife to create a small garlic and salt pile, then press and scrape again. Repeat this process a few more times until you have a smooth paste. Sprinkle ½ teaspoon black pepper over the paste.

Trim the silverskin from the pork loin and rub with garlic paste. Trim the silverskin from 1 (3- to 4-pound) boneless pork loin roast, but leave the fat cap intact. Rub the garlic paste evenly over all sides of the pork loin. Let sit at room temperature for 30 minutes or refrigerate for up to 1 day.

Brown the pork loin on all sides. Heat 2 tablespoons olive oil in a 10- to 12-inch cast iron pan or stainless steel straight-sided skillet over medium-high heat until shimmering. Add the pork and sear on 4 sides until deep golden-brown and it easily releases from the pan, 4 to 6 minutes per side. Transfer to a plate.

Add the butter, onion, apples, and herbs to the pan. Turn off the heat, but keep the skillet on the stove. Scatter apples, onion, butter, 2 fresh rosemary sprigs, and 2 fresh sage leaves around the pan. Drizzle with ¼ cup maple syrup.

Return the pork loin and any juices to the pan. Nestle the browned pork loin on top of the onion, apples, and herbs, and pour in any accumulated juices from the plate.

Roast the pork loin for 1 hour. Roast the pork loin until an instant-read thermometer inserted into the thickest part registers at least 145°F, about 1 hour. Meanwhile, make the green apple sauce.

Make the green apple sauce while the pork roasts. Prepare the following, adding each to the bowl of a food processor fitted with the blade attachment as you prepare it: Peel, core, and quarter 1 medium Granny Smith apple. Finely grate the zest of 1 medium lemon until you have ½ teaspoon, then juice the lemon until you have 1 tablespoon. Mince 1 garlic clove. Finely chop fresh mint leaves and fresh sage leaves until you have a ¾ teaspoon of each. Finely chop fresh parsley leaves and fresh rosemary until you have a ½ teaspoon of each. Add ¼ teaspoon kosher salt, and pulse, scraping down the sides of the bowl as needed, until finely chopped, 12 to 14 pulses. Drizzle in ¼ cup olive oil and pulse again until incorporated.

Rest the pork loin for 20 minutes. When the pork loin is ready, transfer to a clean cutting board and let rest for 20 minutes.

Slice the pork loin and serve. Cut the pork loin crosswise into ¾-inch-thick slices. Serve with the onion and apple pieces, drizzled with some of the pan drippings and the green apple sauce.