Rinse pork and pat dry. Sprinkle all over with salt and pepper. Place in a 4- to 6-quart Dutch oven or other heavy pan with a lid. Add tomatoes, onions, chilies, and garlic.
Cover and bake in a 250° regular or convection oven until meat is very tender when pierced with a fork, 3 1/2 to 4 hours. After the first 2 1/2 hours, check every half hour; if mixture appears dry, add 1/2 cup water to pan.
With two large forks, pull meat from bone into large chunks. Transfer pork to a serving platter. Pour pan juices into a bowl to spoon over pork.