Heat the oil in a large saucepan set over medium-high heat; add the onion, garlic, lemon zest, and pepper, and saute until the onion is softened, about 3 minutes. Stir in barley and stock and bring to a simmer.
Reduce heat; cover pan and simmer for 45 minutes, stirring vigorously halfway through for 15 seconds. Stir in shrimp and peas; cover pan and simmer until shrimp are pink and peas are tender, about 5 minutes. Sprinkle with parsley or mint and serve.