Preheat the oven to 180°C.
Peal and the garlic and onion. Quarter the onion. Place the tomatoes, onion and garlic on roasting tray.
Once the tomatoes blister on top, after about 5-10 minutes, flip them around and cook them until they blister up again. Remove from the oven.
Once the tomatoes have cooled slightly, peel the skins off and cut out any stalky bits. Squeeze out any excess liquid with a sieve.
Place the roasted tomatoes, garlic and onion in a blender with the olive oil, basil and red wine vinegar. Blend until very smooth, then taste and season with salt and pepper.
Store the passata in a freshly washed jar, top with a little more olive oil and keep in the fridge.