Huê´-Style Spicy Beef and Rice Noodle Soup (Bún Bò Huê´)

Ingredients

• 7 tbsp. canola oil
• 3 medium yellow onions, 2 cut into 1" dice, 1 thinly sliced
• 1 tbsp. annatto seeds
• 2 lb. boneless beef shank (shin), halved crosswise, tendon removed and discarded
• 1 lb. boneless pork leg, from the upper butt portion
• Salt
• freshly ground black pepper
• 4 lb. beef bones, cut into 2" pieces, boiled for 3 minutes
• 1 1⁄2 lb. fresh pork hock, cut into 1⁄2" slices
• 3 tbsp. plus 2 1⁄2 tsp. Vietnamese fish sauce
• 5 trimmed stalks lemongrass, 4 cut into 3" pieces, bruised;1 minced
• 1 1" chunk Chinese yellow rock sugar
• 3 tbsp. dried chile flakes
• 3 cloves garlic, minced
• 1 tsp. granulated sugar
• 2 tbsp. fine shrimp sauce
• 2 (14-oz.) packages large round bún (Vietnamese rice noodles), boiled, then rinsed with cold water
• 1⁄3 cups chopped rau ra˘m (Vietnamese coriander)
• 3 scallions, green parts only, trimmed and thinly sliced