How to Make It
Preheat the oven to 275°. Season the pork with salt and pepper. In a medium-size nonreactive oven-safe pot, melt the butter. Add the pork, and cook over moderately high heat, turning occasionally, until browned all over, about 10 minutes. Add the milk, zest, sage, thyme, bay leaves, and baking soda; cover and bring to a simmer. Transfer to the oven, and cook, covered, turning the pork a few times, for 2 hours and 40 minutes. Using tongs, carefully lift the pork and place the leeks underneath. Return to the oven and cook, uncovered, until the leeks and pork are very tender, about 20 minutes.
Transfer the pork and leeks to a work surface. Discard the herbs and zest. Pour pan juices, which may look curdled, into a blender, and puree until smooth. Season sauce to taste with salt and pepper.
Remove the strings from the pork, and thinly slice the meat. Serve pork slices with the leeks and sauce.