Nyonya style fish achar

Ingredients

• 10 sardine fish (or other firm fish ; de-gut and keep the head intact)
• 1 inch fresh turmeric (kunyit ; smashed)
• 10 shallots (remove skin)
• 1 inch ginger (cut into stripes)
• 6 birds/red chillies (6-7, slice red chillies)
• 1/2 cup vinegar
• 6 tbsp sugar (6-8)
• Coriander leaves (optional)
• Salt to taste
• Oil (as needed)