Preheat the oven to 220°C or 190°C fan forced.Prepare all your vegetables: Chop the large tomatoes into smaller pieces, roma tomatoes in half and leave cherry tomatoes whole. Peel the onions, cut in half and then cut each half into 4 wedges. Peel the garlic cloves. Cut the red capsicum into 6 pieces, discarding the core and seeds. Peel the potato and cut into small cubes.Place all the prepared vegetables onto 1 or 2 oven trays so they can be spread out evenly. If using 2 trays, use the fan forced option on your oven.Add the tomato paste, salt, sugar and olive oil to the vegetables and using clean hands, massage this into the vegetables so everything is coated well with olive oil etc. Grind over black pepper. Spread the vegetables evenly over your tray/s and place trays in the preheated oven.
Roast the vegetables for 20 minutes. After this time, remove from the oven, give the vegetables a gentle stir to move them around a bit, then spread evenly and place back in the oven for a further 20 minutes, or until the vegetables have softened and the potato cubes are cooked through. You also want to see a bit of colour/caramelisation on some of the edges of the vegetables.
Remove from the oven and allow the vegetables to cool on the trays for 15 minutes (remove gently from the oven as there will be juices on the tray too). If using capsicum, remove their skin once the vegetables have cooled down (optional but i like to do this).
Carefully transfer the cooled vegetables and their juices to the bowl of a food processor (do this in 2-3 batches) and process until mixture is smooth and creamy. Transfer to a large pot. If you prefer, you can transfer the vegetables straight to the pot from the oven tray and use a stick blender to blend the soup. Once the vegetables are all blended, add the stock to the pot and reheat gently, stirring, over a low heat.
Check for seasoning once the stock is added. Your soup is now ready to enjoy!