Preheat the oven to 190°C/375°F and position one rack in the middle of the oven.
Place the tomatoes, bell pepper and onions in a large baking dish. Drizzle with olive oil, sprinkle with salt, pepper and garlic powder, and toss to coat. Then turn the tomatoes and bell pepper so that they are skin down in the dish.
Bake for about 45 minutes, or until the tomatoes start collapsing and the onions start caramelizing.
Empty the baking dish in a deep saucepan and puree with an immersion blender. If you use a conventional blender or food processor, work in batches to puree.
Blend in 250ml/1 cup of the stock and keep adding the rest about half a cup at a time until the soup is the desired consistency. I use 500ml/2 cups of broth, but add more if you like a thinner soup.
Stir in the sugar, then add smoked paprika to taste.