Heat oven to 200C/180C fan/gas 6. Place the peppers on a baking tray and roast in the oven for about 45 mins until the skins are blackened.
Remove from the oven and put into a plastic bag – this makes them sweat and the skins slip off more easily. When cool enough to handle, peel off the skins with your fingers.
Slice the peppers open, pick out and discard all the seeds and membrane, then roughly chop the red flesh.
Heat the olive oil in a frying pan. When hot, fry the celery and carrot for a few mins.
Add the chopped peppers and continue to fry for a few mins, stirring to combine everything.
Add the vegetable stock, garlic infused oil and tomato puree , bring to the boil, then allow it to reduce a little.
Pour the contents of the pan into a blender and whizz until smooth. Adjust the seasoning to taste.
Depending on the ripeness of the peppers, you shouldn’t need any sugar – but if they retain a slightly bitter flavour, return the sauce to the pan, add sugar to taste and let it dissolve over the heat. Serve hot or at room temperature.