Heat oil in a cooking pot and saute onion garlic and green chillies (all chopped). Pour the prawn stock into the pot and add some soup cubes to give it a taste. Stir well. Add salt, black pepper powder, sliced carrots, chopped red and yellow bell peppers and blanched mushrooms. Add some boiled rice noodles and stir again. To the boiling stock, add prawns. Mix well and cook for three minutes. Transfer it to a soup bowl. Sprinkle some crushed black pepper and serve.