Step 1

Cut pork into 3-inch pieces, and place in a large ziplock bag. Sprinkle with brown sugar, 1/2 teaspoon salt, and 1/2 teaspoon black pepper. Add 2 tablespoons lime juice and 2 tablespoons orange juice to bag, and seal, removing as much air as possible. Refrigerate 8 to 24 hours.

Step 2

Place 2 cups hot water and dried ancho chile in a bowl, and let stand 10 minutes. Drain, reserving 1/4 cup soaking liquid. Transfer pork to a 5- to 6-quart slow cooker, and discard marinade. Add garlic, sliced onion, bay leaf, cinnamon stick, oregano, ancho chile, reserved 1/4 cup soaking liquid, remaining 2 tablespoons lime juice, and remaining 2 tablespoons orange juice. Cover and cook on low until meat is very tender, 7 to 8 hours.

Step 3

Remove pork to a bowl, and pour cooking liquid through a fine wire-mesh strainer into a separate bowl. Discard solids. Heat canola oil in a large cast-iron skillet over medium-high. Add pork, and sear 1 minute without stirring. Return pork to bowl. Add 1/2 cup cooking liquid to skillet. Use a wooden spoon to scrape up any bits of pork stuck to bottom of skillet. Pour over pork; add remaining 1/2 teaspoon salt and remaining 1/4 teaspoon pepper. Shred pork. Divide pork evenly among tortillas; top each with 1 1/2 teaspoons chopped onion. Serve with cilantro and lime wedges, if desired.