1. Heat a large saucepan over high heat. Add broth to pan; bring to a boil. Stir in orzo; reduce heat, and simmer, partially covered, until pasta is al dente and most of the liquid is absorbed (about 10 minutes), stirring occasionally.
2. Heat butter in a medium nonstick skillet over medium heat; stir in shallot and zest. Cook until shallot is softened (about 3 minutes), stirring frequently. Add peas; cook until peas are bright green (about 2 minutes), stirring occasionally. Remove skillet from heat; stir in lemon juice, chopped tarragon, salt, and pepper. Transfer pea mixture to a plate.
3. Add shrimp and orzo to skillet; cook until heated through (about 4 minutes), stirring frequently. Add Parmesan to pan; stir until creamy. Stir in pea mixture. Transfer to 4 bowls, and garnish with tarragon leaves and thinly sliced lemon zest, if desired. Serve immediately.