Milk? Yes, milk! It caramelizes to make a rich, nutty sauce. It’ll also curdle, but it’s supposed to. Whole milk is key for the sauce to work, so skip the skim.
Tie kitchen twine around the pork at 2-inch intervals. In a small bowl, mix 1 tbsp. chopped sage, the salt, coriander, cumin and 1 tsp. pepper; rub on the pork. Wrap in plastic; refrigerate overnight. Unwrap; let standat room temperature 30 minutes.
In a large pot with a lid, heat the oil over medium-high. Add the pork, fat side down. Cook until browned all over, turning occasionally and lowering the heat if beginning to burn, about 10 minutes. Transfer the porkto a plate. Pour off all but 1 tbsp. drippings from the pot; add the onion and garlic. Reduce the heat to medium and cook, stirring occasionally and scraping up the browned bits from the bottom of the pot, until the onions are soft and translucent, 2 to 3 minutes.If there are still some browned bits stuck on the bottom of the pot, add 1/2 cup milk and stir, scraping up the bits. Add the remaining 2 tsp. chopped sage, the marjoram, bay leaves and the remaining 3 1/2 cups milk.
Bring to a gentle boil over medium heat. Return the pork to the pot and reduce the heat to low. Cover with the lid slightly ajar and simmer until an instant-read thermometer inserted into the center of the pork registers 160°, 2 to 3 hours. Transfer the pork to a plate and tent with foil.
Discard the bay leaves. Bring the sauce to a gentle boil over medium heat. Simmer, stirring frequently, until the sauce is nut-brown with dark curds, 20 to 30 minutes; season.
Carve the roast into 1/2-inch slices. Spoon the sauce on the on the plate, reserving some to pass at the table. Arrange the pork on top of the sauce.