Bring a large saucepan of water to a boil over high heat. Add orzo and cook for 7 minutes. Add shrimp and peas and cook until the peas are tender and the shrimp are cooked through, 2 to 3 minutes more. Drain.
Meanwhile, whisk oil, lemon zest and juice, dill, shallot, mustard, salt and pepper in a large bowl.
Add the orzo, shrimp and peas to the bowl and toss to combine. Serve warm or at room temperature and garnished with more dill, if desired.
To make ahead: Refrigerate for up to 1 day.