Heat the chicken broth in a saucepan over medium-high heat until very hot and then reduce the heat to keep the broth hot. In another wide, heavy saucepan, melt 2 Tbs. of the butter over medium heat. Add the onion and a generous pinch of salt and sauté, stirring occasionally with a wooden spoon, until the onion softens and starts to turn lightly golden, 3 to 5 min. Add the rice and stir until the grains are well coated with butter and the edges become translucent, 1 to 2 min. Pour in the wine and stir until it’s absorbed, about 1 min. Add another generous pinch of salt and ladle enough of the hot broth into the pan to barely cover the rice, about 1 cup. Bring to a boil and then adjust the heat to maintain a lively simmer. Cook, stirring occasionally, until the broth has been mostly absorbed, 2 to 3 min. Continue adding broth in 1/2-cup increments, stirring and simmering, until it has been absorbed each time, at intervals of about 2 to 3 min.
After about 16 to 18 min., the rice should be creamy but still fairly firm. At this point, add the peas and another 1/2 cup broth. Continue to simmer and stir until the peas are just cooked and the rice is just tender to the tooth, another 3 to 4 min. Stir in another splash of broth if the risotto is too thick. Remove the pot from the heat and stir in the mint, lemon juice, lemon zest, the remaining 2 Tbs. butter, and the Parmigiano. Season with salt to taste. Serve the risotto immediately with a sprinkling of Parmigiano.