Preheat the oven to 220°C, 425°F, Gas Mark 7.
Place the tomatoes in a large roasting in with the unpeeled garlic cloves and chilli and drizzle lightly with olive oil. Roast in the oven until the skins are blackened and the tomatoes are soft – about 40 minutes. Leave to cool.
Remove the skins from garlic, then place the contents of the roasting tin in either a pestle and mortar if you have one, or a food processor. Add the onion and lime juice and either pound or pulse until you have a rough consistency – not too smooth.
Add the coriander leaves and season.