1Spray bottom of 5- to 6-quart slow cooker with cooking spray. Line side of slow cooker with foil; spray foil with cooking spray. Cook and drain pasta as directed on package, using minimum cook time.
2Meanwhile, in 12-inch skillet, cook beef and onion over medium heat 8 to 10 minutes, stirring occasionally, until beef is thoroughly cooked; drain. Stir in spinach, oregano, salt and pepper. In medium bowl, mix pasta sauce and sugar.
3In slow cooker, spread 1 1/2 cups of the pasta sauce. Layer with half of the pasta, half of the beef mixture and 1 cup of the feta cheese. Repeat layers. Pour remaining 1 1/2 cups pasta sauce over top. Sprinkle with mozzarella cheese.
4Cover; cook on Low heat setting 3 hours or until bubbly. Let stand 10 minutes before serving. Garnish with basil.