In a small saucepan, heat the chicken broth and 1 3/4 cups water over high heat until steaming; cover and keep warm. Meanwhile, in a medium saucepan, melt 1 tbsp. butter over medium heat. Add the onion and cook, stirring occasionally, until softened, about 5 minutes. Add the rice and 1/2 tsp. salt and cook, stirring, for 3 minutes.
Stir in 1 cup of the broth mixture, bring to a simmer and cook, stirring, until the liquid is absorbed, 2 to 3 minutes. Maintaining a low simmer, continue to add the broth mixture, 1/2 cup at a time, stirring frequently and allowing the rice to absorb the liquid. Cook until the rice is tender and creamy, 20 to 25 minutes. Add the remaining 2 tbsp. butter, the shrimp, peas, and 1/2 cup cheese and cook, stirring, until the cheese has melted. Stir in the lemon zest; season with salt and pepper. Top with the remaining 1/2 cup cheese.