Tip:To wilt the spinach perfectly, put it in a colander and drain the pasta over it.
Position a rack in the center of the oven and heat the oven to 350°F. Meanwhile, bring a large pot of generously salted water to a boil. Put the spinach in a colander in the sink.
Toast the walnuts on a rimmed baking sheet in the oven until dark golden-brown, 6 to 8 minutes. Set aside.
Cook the penne according to package directions. Reserve about 1/2 cup of the cooking water and drain the pasta in the colander over the spinach.
Put the cream, Gorgonzola, 1/4 tsp. pepper, and a pinch of salt in a 12-inch nonstick skillet and bring to a boil over medium-high heat. Cook, stirring frequently, until slightly thickened, 2 to 3 minutes. Reduce the heat to medium, add the pasta and spinach, and 2 Tbs. each of the walnuts and chives. Cook, stirring constantly, until some of the sauce is absorbed by the pasta, about 2 minutes. If necessary, add some of the cooking water to moisten the pasta. Season to taste with salt and pepper. Transfer to serving bowls, sprinkle with the remaining 2 Tbs. walnuts and 1 Tbs. chives, and serve.