Put a kettle on to boil. Heat the stock to simmering point in a small saucepan. Meanwhile, in a larger saucepan, heat the olive oil then add the onions and fry them over a medium heat for 5 minutes, stirring frequently, until tender.
Add the pearl barley to the pan and stir continuously for 1-2 minutes until the grains are evenly coated with the oil and onion mixture. Increase the heat, add the wine or sherry to the pan and simmer for 1-2 minutes until the barley has absorbed the liquid.
Add a ladle of the hot stock to the barley, stirring continuously until it has been absorbed. Continue to add the stock a ladle at a time, stirring, and add half the peas before the last few ladles of stock. Continue stirring frequently until all the stock has been absorbed and the barley is al dente. This takes 35-45 minutes.
Meanwhile, place the remaining peas in a bowl and pour 100 ml of boiling water over them, or just enough to cover them. Leave them to stand for 1-2 minutes, then purée them with their liquid using a food processor or a hand-held mixer.
Just before serving, stir the puréed peas and Parmesan into the orzotto and quickly heat them through. Season with salt and pepper to taste.
using barley gives this mixture a slightly firmer, more granular texture than a risotto made with rice, as barley grains do not release much starch during cooking.