Step 1

In a medium saucepan over medium, bring the broth and 2 cups water, covered, to a simmer; reduce to low to keep warm.

Step 2

In a large saucepan over medium, melt 2 tablespoons butter. Add the onion and ¼ teaspoon salt; cook, stirring occasionally, until softened, about 5 minutes. Add the rice and cook, stirring constantly, until the grains are translucent at the edges, 1 to 2 minutes. Add 3 cups of the hot broth mixture and bring to a boil over medium-high. Reduce to medium and cook, stirring often and briskly, until most of the liquid is absorbed, 10 to 12 minutes; adjust the heat as needed to maintain a vigorous simmer.

Step 3

Cook, adding ¼ cup of the broth at a time, until the rice is al dente and loose but not soupy, another 8 to 10 minutes. You may not need all of the broth. Off heat, stir in the remaining 1 tablespoon butter, the peas, lemon zest and juice, and Parmesan. Taste and season with salt and pepper. Serve sprinkled with parsley and additional Parmesan.