Preheat the oven to 180°C.
Cut the tomatoes in half and put them a large baking tray.
Roughly chop the celery and carrot. Peel and cut the onion into thin wedges. Peel the sweet potato then dive into 3 cm chunks. Put the chopped veggies around the tomatoes. Scatter the garlic cloves around the tomatoes.
Drizzle with the oil and dust with the oregano and chilli flakes (if used). Season with salt and pepper. Combine well, then bake for 25-30 minutes until the vegetables have softened.
Transfer the roasted vegetables to a saucepan, including the cooking jus. Add the stock and bring to the boil.
Turn off the heat, add the fresh herbs and the vinegar.
Using a hand-held blender, whiz the soup until you get a smooth texture. Add more stock if it’s too thick.
Serve warm with freshly ground pepper and a pinch of dried chilli flakes.