Arrange a rack in the middle of the oven and heat the oven to 350 degrees. Place the pork in a Dutch oven or other large ovenproof pot with a lid and season all over with salt and pepper.
In a small bowl, whisk together the mustard, vinegar, brown sugar and garlic until the sugar has dissolved. Pour the mixture all over the pork chunks and rub it in with your hands. Without pouring it directly over the pork, add 2 cups of water to the pot.
Cover the Dutch oven and bake for 1¼ hours. Adjust the oven temperature to 450 degrees and carefully uncover the pot. Bake until almost all of the liquid has evaporated (leaving mostly rendered fat) and the pork is golden brown and tender, about 30 minutes more.
Serve the pork as desired, with beans, salad greens and a dollop of mustard, if you like.
