Add the olive oil and onion to a large frying pan and heat to a medium heat. Fry for approx 8-10 minutes until soft and browning.
Char-grill the peppers on a griddle pan (high heat) for approx 10 minutes, turning frequently until nice and charred on both sides.
Set aside to cool, then chop into small pieces.
Add the garlic into the frying pan with the onion and stir for 2-3 minutes, then add in the chopped tomatoes and stir to combine.
Add a splash of water, the tomato purée, sundried tomatoes and the chopped roast peppers, then simmer for 10 minutes.
Season with salt, pepper and chilli flakes (if using)