Slice green onions; separate dark-green parts from white and pale-green parts.
Heat oil in a large (5- to 7-qt.) Dutch oven or other heavy pot over medium heat. Add white and pale-green parts of green onions; cook, stirring often, until just softened, about 2 minutes. Mix in bell pepper; cook, stirring often, 2 minutes. Add rice; stir to coat in oil. Stir constantly, until toasted, about 1 minute. Add water, salt, and black pepper; bring to a boil over medium-high. Reduce heat to medium-low; cover and simmer, 15 minutes.
Stir in shrimp. Cover and cook over medium-low heat until shrimp are pink, about 5 minutes. Remove from heat.
Scatter peas over risotto. Cover; let stand, 10 minutes. Uncover and stir in peas.
Garnish with dark-green parts of green onions. Divide among four bowls; serve with lemon wedges.
Arborio rice (not butter and cheese) gives this risotto its creamy texture.