Spanish Mackerel with Yuzu

Bahan-bahan (2 porsi)

• 2 fillets Spanish mackerel (sawara) ((½ lb, 227 g))
• ¼ cup soy sauce ((4 Tbsp))
• ¼ cup mirin ((4 Tbsp))
• ¼ cup sake ((4 Tbsp))
• 1 tsp yuzu zest ((or lemon zest))
• 1 cup water
• ⅓ cup sugar
• 4 kumquats ((kinkan))