Remove small bones from fish; cut fillets in half crosswise and slice into long, thin strips. Place in a large bowl with about 300ml rice vinegar, making sure fish is covered with liquid. Marinate 1 hour.
Place onions, sugar, salt and remaining vinegar in a bowl. Stir until sugar and salt have dissolved. Leave to soak 30 minutes.
Crush peanuts finely. Dry–fry in a heavy nonstick pan over low heat 2 to 3 minutes until evenly browned. Remove from pan and set aside.
Add rice to pan and toast 3 to 4 minutes until golden brown all over, then crush finely.
Remove marinated fish and onion from vinegar. Place in a large bowl and add mint and chili. Toss gently to combine; sprinkle toasted crushed peanuts and rice on top and toss again. Garnish with extra mint and chili.