Preheat oven to 400°F / 200°C. Cut the bell peppers in half and remove the ribs and seeds. Place them cut-side down on a lined baking sheet and roast for 35 to 40 minutes until they are charred in a few places.
Remove the peppers from the baking sheet and transfer to a bowl and quickly cover them with a plate to trap the steam (this makes the peppers very easy to peel!). When the peppers are cool enough to handle, remove and discard the skins.
Place the peppers in a blender with all other ingredients, and blend until smooth. Taste and adjust seasoning as desired. Store leftovers in the fridge for up to one week.