Step 1
In a small bowl, mix oil, garlic, fennel seeds, 3 tsp sea salt (flakes preferred) and 1 tsp freshly ground black pepper. Rub over chickens. Cover and refrigerate for at least 1 hour and up to 1 day.
Step 2
To make blackened chilli vinegar, prepare a covered barbecue for indirect cooking. For a gas barbecue, heat burners under the grate side on medium-high heat; leave burners under the flat plate off. For a charcoal barbecue, build a fire on one side of charcoal grate, leaving other side of barbecue empty, and let burn just until coals are covered with white ash (do not spread out coals).
Step 3
Toss chillies with olive oil. Place over lit side of barbecue. Cook, uncovered, turning as needed, for 8-10 mins or until chillies are charred all over. Cool slightly on a chopping board. Close barbecue lid and leave on to preheat. Using a small knife, remove stems and seeds and scrape away some of loosened skin from chillies (leave some blackened skin intact). Coarsely chop. In a medium bowl, combine chillies, vinegar, extra virgin olive oil, shallots, sugar and 2 tbs salt. Set aside. (Refrigerate, covered, for up to 3 days; serve at room temperature.)
Step 4
Place chickens on a wire rack set in a large rimmed baking tray. Place tray of chickens over unlit side of barbecue. Close barbecue hood and cook, turning tray 180 degrees halfway through cooking, for 1½ hours or until chickens are dark golden and an instant-read thermometer inserted into thickest part of thighs registers 74°C. Rest chickens on a carving board.
Step 5
Carve chickens and transfer to platters. Sprinkle with oregano and serve with blackened chilli vinegar.