Place barley in a bowl, and add enough cold water to cover by 2 inches. Cover; let soak in the refrigerator overnight.
Bring stock to a boil in a saucepan. Remove from heat, and cover. Drain barley. Heat oil in a medium saucepan over medium heat. Add onion; cook 6 minutes. Stir in barley. Add wine, and cook, stirring, until liquid has evaporated.
Add 1/2 cup stock, and cook, stirring, until liquid is absorbed. Continue adding stock, 1/2 cup at a time, waiting until liquid is absorbed before adding more, until barley is tender but still slightly firm, about 35 minutes total (reserve 1/4 cup stock to stir in at the end).
About 1 minute before barley is done, stir in shrimp and peas, and cook until shrimp are opaque throughout. Remove from heat, and stir in 1/4 cup stock, lemon zest, lemon juice, Parmesan, and salt. Season with pepper, and serve.