8blue mackerel fillets, pin-bonedFor drizzling:olive oil½Spanish onion, thinly sliced1½ tspsumac4soft white rolls1 cup(loosely packed) corianderTo serve:lime cheeksLemon-lime aioli2egg yolks1 tbspSherry vinegar1½ tspDijon mustard1garlic clove, finely chopped80 ml (1/3 cup)vegetable oil70 mlextra-virgin olive oilJuice of1 lemon and ½ lime
For lemon-lime aioli, process yolks, vinegar, mustard and garlic in a food processor to combine, then gradually add oils in a thin steady stream, processing until thick and emulsified. Season to taste, add lemon juice and lime juice, refrigerate until required. Makes about 200ml. Remaining aioli will keep refrigerated for 1 week.
Preheat a barbecue or char-grill pan to high heat. Drizzle fish with oil, season with sea salt, and barbecue skin-side first, turning once, until just cooked (2 minutes each side).
Meanwhile, combine onion and sumac in a bowl and set aside. Halve rolls, spread bases generously with lemon-lime aioli, then divide fish among rolls. Top with onion mixture and coriander, season to taste, sandwich with tops and serve with lime cheeks.