Preheat oven to 350 degrees. Coat a 2-quart baking dish and small nonstick skillet with cooking spray. Cook pasta according to package directions. Drain well; set aside.
Meanwhile, in the small skillet, cook onion over medium heat until tender, 5 minutes, adding spinach during the last 2 minutes of cooking. Cool.
In large bowl, combine spinach and onions with ricotta, egg, seasoning, and zest.
In another bowl, combine Alfredo sauce, water, and lemon juice.
Spread 1/2 cup sauce mixture in bottom of baking dish. Top with half of the pasta, half of the spinach mixture and half of the remaining sauce; repeat.
Cover with foil and bake until bubbly, about 30 minutes. Uncover and bake until lightly browned, about 15 minutes, sprinkling with feta cheese during the last 5 minutes of cooking. Cool 10 minutes before serving.