Preparation Bring broth to a simmer in a saucepan, then keep at a simmer. Meanwhile, turn on a food processor and drop in garlic, processing until finely chopped. Turn off processor, then add onion and pancetta and pulse until finely chopped. Cook onion mixture with zest in oil and 1 tablespoon butter in a 12-inch heavy skillet over medium-high heat, stirring once or twice, until onion is softened and just beginning to turn golden, about 6 minutes. Cook peas in broth 2 minutes. Strain broth into onion mixture, reserving peas, then stir in rice. Cook, covered, over high heat, stirring once after 5 minutes, until rice is al dente, 10 to 12 minutes. Stir in peas, cheese, remaining tablespoon butter, and 1/4 teaspoon pepper.