Halve, peel and finely chop the onion. Peel and finely chop the carrot. Finely chop the celery. Warm a pan over a medium heat. Drizzle in 1 tbsp oil and sprinkle in 2 tsp fennel seeds. Add the veg and cook on a medium-low heat for 7-8 mins. Stir in the barley. Add 300ml boiling water. Simmer for 25- 30 mins till the barley is just soft and the water is mostly absorbed.
Heat the grill to its highest setting. Halve the peppers lengthways. Pull off the stalk and remove the pith and seeds. Place the peppers skin-side up on a foil-lined baking tray. Place under the grill for 8-12 mins till the skin is blackened. Carefully remove the peppers and place them in a bowl. Cover with cling film and leave to steam.
Peel and thinly slice the red onion. Put it in a small bowl. Add the raisins. Cover with boiling water. After 5 mins, drain well.
Set a frying pan on a medium heat. Tip in the pine nuts and toast for 2-3 mins, shaking the pan now and then. Tip into a pestle and mortar. Peel and crush the garlic. Finely shred the sorrel, stalks and leaves. Add to the pine nuts with 1 tbsp oil and a pinch of salt. Pound together to make a paste. Alternatively pulse in a food processor or finely chop everything.
Trim the radishes, reserving the leaves. Finely slice the radishes. Roughly shred the leaves. Place into a large serving bowl. Cut the cucumber into quarters lengthways. Cut out and discard the seeds. Chop the cucumber into small chunks and add to the bowl with the radishes.
Remove the peppers from the bowl. Peel off and discard the papery blackened skin. Cut the peppers into thin ribbons.
Drain off any excess water from the barley and shake it dry in a sieve. Tip it back into the pan. Zest in the orange. Add 1 tsp sorrel pesto and stir to coat the grains. Taste and season.
Transfer the red onion and raisins to the veg bowl. Pour in the cooked barley. Stir everything together. Top with the pepper ribbons and dollops of remaining sorrel pesto. Serve with wedges of orange to squeeze over.