When a pork loin is braised in milk Bolognese-style, the meat becomes tender and very finely textured. The milk (here, aided by cream) morphs into a luxurious sauce.
Season pork with 1 tablespoon salt and 1/2 teaspoon pepper. Heat oil in a heavy-bottomed pot (it should be just large enough to fit pork) over medium-high heat. Add pork, fat side down, and sear, turning as it browns, until browned on all sides, 8 to 10 minutes. Transfer to a plate. Drain fat from pot and wipe out any blackened bits.
Tie together sage, rosemary, and bay leaves into a bouquet with kitchen twine. Melt butter in pot over medium heat. Add herbs, garlic, and lemon zest and cook until fragrant, about 30 seconds. Return pork, fat side up, to pot, along with any accumulated juices. Pour milk and cream over pork, raise heat to medium-high, and bring to a simmer. Partially cover pot, reduce heat, and simmer, undisturbed, until a thermometer inserted into thickest part of pork registers 140 degrees, about 1 hour.
Transfer pork to a carving board and let rest at least 15 minutes and up to 30 minutes. Increase heat to medium and simmer braising liquid until reduced to 1 1/2 cups, 12 to 15 minutes. Discard herbs and lemon zest. Cut pork crosswise into 1/2-inch-thick slices and serve with milk gravy.