This recipe works best prepared with a pork roast that is about 7 to 8 inches long and 4 to 5 inches wide. We found that leaving a ¼-inch-thick layer of fat on top of the roast is ideal; if your roast has a thicker fat cap, trim it back to be about ¼ inch thick. To prevent the spices from burning when browning the pork in step 2, be sure to use medium heat.

Adjust an oven rack to the lowest position and heat the oven to 250 degrees. Pat the pork dry with paper towels. Toss the coriander, paprika, salt, brown sugar, anise, cumin, and cayenne together in a small bowl, then rub the mixture evenly over the pork.

Heat 2 tablespoons of the oil in a large Dutch oven over medium heat until just smoking. Lightly brown the pork on all sides, 5 to 7 minutes, reducing the heat if the pot begins to scorch or the spices begin to burn. Transfer the pork to a large plate.

Add the remaining 1 tablespoon oil to the pot and heat over medium heat until shimmering. Add the onions, cover, and cook until softened and wet, about 5 minutes. Remove the lid and continue to cook the onions, stirring often, until dry and well browned, 10 to 12 minutes. Stir in the garlic and cook until fragrant, about 30 seconds.

Off the heat, nestle the pork, along with any accumulated juices, into the pot. Place a large sheet of foil over the pot and press to seal, then cover tightly with the lid. Transfer the pot to the oven and cook until the very center of the roast registers 140 to 145 degrees on an instant-read thermometer, 35 to 55 minutes.

Remove the pot from the oven. Transfer the pork to a cutting board, tent loosely with foil, and let rest until the center of the roast registers 150 degrees on an instant-read thermometer, about 20 minutes. Stir the butter into the onions, season with salt and pepper to taste, and cover to keep warm.

Remove the twine, slice the pork thin, and transfer to a serving platter. Spoon the onions over the pork and serve.