Cook fettuccine according to package directions, adding green peas during the last 2 minutes of cook time. Reserve 1/2 cup pasta cooking water to thin sauce, if necessary.
Meanwhile, heat olive oil in 12-inch nonstick skillet over medium-high heat and cook chicken, stirring occasionally, until chicken is almost done, about 5 minutes . Remove chicken with slotted spoon and set aside.
Cook onion in same skillet over medium heat, stirring frequently, until onion is tender, about 5 minutes. Stir in pumpkin and cook, stirring frequently, until pumpkin is slightly darkened, about 2 minutes. Stir in Sauce. Bring to a boil. Reduce heat to low and stir in fettuccine; toss to coat. Stir in sage and nutmeg. Thin sauce, if necessary, with pasta cooking water.