In a bowl, mix together pork and prawns using a fork, then thoroughly mix in garlic, coriander, spring onions (scallions), half of egg, the fish sauce and black pepper
Divide between bread, spreading it firmly to edges. In a small bowl, stir together remaining egg and coconut milk and brush over pork mixture. Trim crusts from bread, then cut each slice into squares.
In a wok, heat oil to 190 c (375 f). Add several squares at a time, pork -side down, and fry for 3 -4 minutes until crisp, turning over halfway through.
Using a slotted spoon, transfer to absorbent kitchen paper to drain, then keep warm in oven. Check temperature of oil in between frying each batch.
Serve warm, garnished with cucumber slices.