Heat the oven to 200C (180C fan) Gas 6. Put the squash into a roasting tin, drizzle with 1tsp olive oil and roast for around 20 minutes until soft and slightly caramelised.
Rinse barley and drain in a sieve. Gently fry onion in the oil and after a couple of minutes add celery, cook for 5 minutes until beginning to soften, turn up heat, add white wine and bubble up vigorously, stirring for a couple of minutes. Stir in the barley, then pour on stock, stir, turn down heat to simmer. You can more or less leave it for the first 20 minutes giving an occasional stir, then check to see if you need to add more liquid - boiling water is fine - while it softens (al dente pearl barley isn't advised) for the last 10 or 15 minutes. If there's too much liquid when the barley is nearly done just boil harder until it's gone. When ready stir in the squash, season with black pepper,sparingly if at all with salt as vegetable stock is salty, and sprinkle over chopped parsley.
Serve with grated Parmesan and a sprinkling of parsley.
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