Preheat the oven to 180 C.
Halve the red peppers. Remove and discard the stems, seeds and white membrane. Chop the tomatoes and onions into thick wedges (approx 2” X 1”). Keep the garlic cloves unpeeled.
Toss them all together with salt and olive oil. Arrange on a large baking tray without too many overlaps. Roast them in two trays if they don’t fit on one. Roast for 60 mins. Set aside until cool enough to handle.
(Note: if your tray is overcrowded this may take longer. Roast until the peppers, onions and tomatoes brown and shrink to half their original size.)
Add the roasted vegetables to a blender jar. Peel the garlic cloves and discard the skins. Add the cashews and chilli flakes and blend to a smooth paste. If using right away, thin out with enough water to get the consistency you want. If storing for later use, store as a thick paste and thin out with water before using.
Decant into jars and refrigerate for up to a week or freeze for up to 3 months.