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 medium 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 more minute.
Heat the broiler. In a large bowl, combine the yogurt, cream cheese, Parmesan, and lemon juice until well blended. Add the onion mixture and stir until combined.
Add the pasta to the bowl and toss to coat. Stir in the spinach, artichokes and 1/4 cup of the pasta cooking liquid. (Add more liquid if the pasta seems dry.) Gently stir in 1/4 cup of the mozzarella.
Transfer the pasta mixture to a broiler-proof 9-inch square casserole dish. Sprinkle with the remaining 1/4 cup mozzarella.