Chop the onions into thick wedges. Quarter the red peppers lengthways and discard the seeds and white membrane. If using cherry tomatoes, keep them whole. If using regular tomatoes, chop into approx 1 inch pieces.
Add to a baking tray along with the garlic cloves (skins on), bay leaf and salt. Toss everything together with olive oil. Pop the garlic cloves under the red peppers.
Roast for 40-45 mins or until the skins on the tomatoes and peppers are blistered in spots. Set aside to cool slightly.
Add all the roasted vegetables to a blender. Add the chilli flakes, peppercorns and basil if using. Squeeze the garlic cloves out of their skins and pop those in too. Discard the bay leaf.
Blend without any water first. Once everything starts to break down, add the milk and enough water to blend to a soup consistency. Check and adjust seasoning. Reheat and serve with a drizzle of olive oil and torn basil leaves on top.
Notes
I use cherry tomatoes here when I can find them. Regular tomatoes or a combination of the two will work just as well too.