Shell peas, reserving pods. Place pods in a medium pot with 8 cups lightly salted water. Bring to a boil over high heat, reduce to medium-low, and simmer for 1 hour. Strain broth into a medium pot, pressing on pods. Discard pods. Keep broth warm over low heat.
Melt 2 tbsp. of the butter in a large heavy-bottomed pot over medium heat. Add pancetta and onions and cook, stirring with a wooden spoon, until onions are golden, about 10 minutes. Add peas and ½ cup warm broth. Cover and cook until peas are tender, 5-10 minutes.
Increase heat to high, uncover, and cook off any remaining liquid. Add oil. When oil is hot, add rice and stir to coat well. Add about ¾ cup warm broth and cook, stirring constantly, until most of the broth has been absorbed. Add about ¾ cup more broth. Continue cooking, stirring and adding broth as needed until rice is tender but firm to the bite, about 20 minutes.
Remove from heat and season to taste with salt and pepper. Stir in 1 more cup broth, remaining 2 tbsp. butter, parsley, and ¼ cup parmigiano-reggiano. Cover and allow to rest for a few minutes. Serve with additional parmigiano-reggiano.