Preheat oven to 200°c. Place pork into a lined roasting pan. Brush with 2 tsp vegetable oil and season. Bake for 10 minutes. Brush with 2 tbs char siu sauce and bake for a further 10 minutes or until cooked through. Rest, covered loosely with foil. Slice.
Meanwhile, combine flours and 1 tsp salt in a bowl. Add 2 tbs vegetable oil, sesame oil and 2 cups water. Whisk until smooth. Stir in onion.
Heat 1 tsp vegetable oil in 2 large non-stick frying pans over medium- high heat. Add 1/4 cup batter to each pan, working quickly to spread each to a 10cm round. Cook for 3 minutes or until golden. Turn and cook for a further 2 minutes or until cooked through. Repeat with remaining batter, adding oil as required, to make 12 pancakes. Cover and keep warm.
Top pancakes with lettuce, pork, cucumber, extra onion and sesame seeds. Drizzle over remaining sauce and serve.