Cut chillies in half and rinse out seeds and membrane under running water. Chop chilly skins finely.
In a large saucepan or stockpot, heat the vegetable oil on high heat.
Sauté the onion until translucent, stirring regularly, (about 4 minutes).
Add the garlic and ginger and cook for a further 1 minute.
Add the eggplant and continue to cook, stirring continuously until the pot is dry and the eggplant starting to colour and soften.
Deglaze the pot with the mirin.
Add the stock and water.
Stir in the brown sugar and miso paste.
Bring to the boil; reduce heat to a low simmer and cook for 2 hours.
Remove from the heat and allow the soup to cool slightly.
Transfer to a food processor or use a hand held stick blender, process the soup until smooth.
Return to heat to warm up.
Taste for seasoning and adjust as required. More miso may be added if desired.
Garnish each serving of soup with a small drizzle of sesame oil, no more than ¼ tsp will be required.