Remove any small bones from the fish, then cut the fillets in half crossways and slice them into long thin strips. Place them in a large bowl and pour over 300 ml of the rice vinegar, making sure the fish is covered. Set aside to marinate for 1 hour.
Place the onions in a bowl, add the sugar and salt and pour over the remaining vinegar. Stir until the sugar and salt have dissolved, then leave to soak for 30 minutes.
Meanwhile, crush the peanuts finely using a pestle and mortar, then dry-fry them in a heavy-based nonstick frying pan over a low heat for 2-3 minutes until they are evenly browned. Remove them from the pan and set aside.
Add the rice to the pan and toast it for 3-4 minutes until it is golden brown all over, then place it in the mortar and crush it finely.
After the fish and onion have marinated, remove them from the vinegar and pat them both dry with kitchen paper. Place in a large bowl and add the mint and chilli. Toss gently to combine, then sprinkle the toasted crushed peanuts and rice on top and toss again. Garnish with extra mint and chilli if you like, and serve.