Preheat the oven to 200°C (fan forced)
Line a tray with baking paper and place the peppers inside. Drizzle lightly with olive oil and roast for about 40-45 minutes at 200°C (might vary according to oven) till the skin of the peppers have blackened.
Remove and cover with a cloth for about 5 minutes; this makes it easier to peel off the skin.
Once the peppers have lightly cooled, remove the skin, pith and seeds.
Place in a food processor along with cashewnuts, garlic, chilli flakes, salt, lemon juice and 3 tbsp olive oil. Blitz well and add more olive oil as needed to get a smooth puree.
If you want a really smooth puree, you can strain the sauce but I skipped the step.