Heat your oven to 200°C/Fan 180°C/Gas 6. Put a large roasting tin on the top shelf of your oven to heat up. Crumble the stock cube into a heatproof jug and dissolve in 500ml boiling water.
Halve, peel and thickly slice the red onion. Cut the pepper into 2cm-thick strips, (discard the stem and seeds). Chop the stem off the chilli. Halve it lengthways and deseed it. Chop the tomatoes into wedges.
Add the onion, pepper and chilli, along with the tomatoes, to the hot roasting tin. Drizzle with 1 tbsp oil. Season. Roast for 20 mins till tender.
While the veg roast, finely dice the celery. Peel and finely chop the garlic. Take the veg out of the oven and stir in most of the celery and all the garlic. Spoon into a blender and add the stock.
Roughly chop the parsley leaves. Add half to the blender. Grate in most of the lemon zest and squeeze in a little juice. Blend till smooth. Taste and season. Add more lemon juice or water if you think it needs it.
Ladle the soup into 2 warm bowls. Top with the remaining parsley leaves, celery and lemon zest, drizzle each one with 1 tsp olive oil and serve.