1.Rinse chickens under cold water; pat dry with absorbent paper. Using kitchen scissors, cut along sides of chickens’ backbones; discard backbones. Halve chickens along breastbones.
2.Blend or process onion, garlic, chillies, vinegar and cumin until almost smooth.
3.Heat oil in a large frying pan, add onion mixture; cook, stirring, until fragrant.
4.Blend or process tomatoes until smooth, add to onion mixture; cook, stirring, until mixture boils. Reduce heat; simmer, uncovered, stirring, for 20 minutes or until thickened. Season to taste. Brush chickens with half the chilli sauce; cover, refrigerate for 3 hours.
5.Preheat oven to 220°C/425°F.
6.Place chickens, skin-side up, on an oiled wire rack in a large shallow baking dish; roast for about 30 minutes or until cooked through.
7.Serve chicken with remaining chilli sauce and, if you like, green salad leaves, and grilled corn and zucchini salsa