In a saucepan, combine broth, 2 cups water, lemon zest, 1 1/4 teaspoons salt, and 1/4 teaspoon pepper. Bring to a simmer over medium heat. Meanwhile, heat a large straight-sided skillet over medium. Swirl in 2 tablespoons oil. Add onion, season with 1/4 teaspoon salt, and cook, stirring occasionally, until soft and translucent, about 5 minutes. Add rice and cook, stirring, until translucent and grains begin to make a popping sound, 3 to 4 minutes. Stir in wine and saffron; cook until wine is mostly evaporated, about 30 seconds.

Ladle 1 cup broth mixture into skillet. Stir with a wooden spoon at a moderate speed until most of broth is absorbed and bottom of skillet stays visible for a second or two as you mix, 2 to 3 minutes. Continue ladling broth into skillet, about 1/2 cup at a time (or enough to just cover rice). Stir slowly but constantly, allowing rice to absorb most of broth before adding more. Risotto is ready when rice is al dente and broth has thickened to a rich sauce, 20 to 25 minutes. (You may not need to use all of broth.) Add shrimp and peas to skillet with last addition of broth; cook until shrimp are opaque, 2 to 3 minutes.

Remove from heat. Stir in butter, lemon juice, and cheese; season with salt and pepper. Drizzle with oil and sprinkle with more cheese and pepper to serve. If the risotto becomes stiff before serving, stir in more hot broth, a little at a time, until it's loose and creamy.