Spinach and Artichoke Baked Pasta

Cook the pasta according to package directions. Reserve 1/2 cup of the cooking liquid, then drain the pasta. Meanwhile, heat the oil in a large skillet over medium heat. Add the onion and 1/2 teaspoon each salt and pepper and cook, covered, stirring occasionally, until tender, 8 to 10 minutes. Add the garlic and cook, stirring, for 1 minute. Heat broiler. In a large bowl, combine the sour cream, cream cheese, Parmesan, lemon zest, and lemon juice; Stir in the onion mixture. Add the pasta to the bowl and toss to coat. Stir in the spinach, artichokes and 1/4 cup of the cooking liquid (adding more liquid if the pasta seems dry). Fold in 1/2 cup mozzarella. Transfer the pasta mixture to a broiler-proof 2 1/2- to 3-quart casserole dish. Sprinkle with the remaining 1/2 cup mozzarella and broil until golden brown, 3 to 5 minutes.