Preheat the oven to 200 C / Gas 6. Line two baking trays with baking parchment.
Arrange tomatoes, onion and red pepper in a single layer on the prepared baking trays. Drizzle with oil and sprinkle with salt and pepper.
Roast in the preheated oven for 30 minutes. Add garlic; continue roasting until vegetables are tender, about 15 minutes more.
Bring stock to the boil in a large pot over high heat. Reduce heat and simmer, covered.
Meanwhile, transfer 1/2 of the tomato mixture into a blender. Cover and pulse 3 times, then blend until smooth, adding hot stock as needed. Pour into the pot with stock. Repeat with remaining tomato mixture, then stir into the pot until combined.
Simmer soup to heat through, about 5 minutes. Stir in basil and parsley.