Heat the broth and 2 1/2 cups water in a saucepan over low heat; keep warm.
Meanwhile, melt 2 tablespoons of the butter in a 3-quart saucepan over medium heat.
Add the zucchini; season with salt and pepper.
Cook, stirring often, until the zucchini is golden, 8 to 10 minutes.
With a slotted spoon, transfer the zucchini to a plate.
Reduce the heat to medium-low.
Add the onion; cook until soft, 5 minutes.
Season with 1 teaspoon salt and 1/4 teaspoon pepper.
Raise the heat to medium.
Add the rice; cook, stirring, until translucent around the edges, about 3 minutes.
Add the wine; cook until absorbed, about 2 minutes.
Cook the rice, adding 1 cup hot broth at a time (stir until almost all the liquid is absorbed before adding more), until tender, 25 to 30 minutes total.
Add the zucchini and peas; cook until the peas are bright green, 2 minutes.
Remove from the heat.
Stir in the remaining tablespoon butter and the Parmesan.