Heat the oven to 200°C/Fan 180°C/Gas 6. Put a roasting tin in the oven to warm up.
Peel and slice the red onion. Tip it into a bowl. Halve the peppers. Scoop out the seeds and white bits. Roughly chop them. Add to the bowl. Quarter the tomatoes and add them.
Peel and grate the garlic and ginger. Finely chop the chilli – flicking out the seeds for less heat. Add the garlic, ginger and chilli to the bowl with the Szechuan peppercorns. Pour in 1 tbsp olive oil and add a pinch of salt. Turn everything to coat it in the oil and spices.
Take the roasting tin out of the oven. Tumble in the veg. Spread them out in an even layer. Roast for 30 mins till the veg are soft and a little charred at the edges. While the veg roast, crumble the stock cube into a heatproof jug. Pour in 600ml boiling water and stir to dissolve.
When the veg are soft, tip them and any juices from the tin into a blender. Pour in the stock. Blitz till smooth. No blender? Pour the stock into the tin and use a hand-held blender to blitz it, or use a potato masher to mash everything together as much as you can, then stir in the stock.
Taste the soup. Add a pinch more salt if you think it needs it. Serve the soup in warm bowls.