Preheat oven to 350°.
Cook pasta shells according to package directions, omitting salt and fat; drain. Rinse under cold water; drain and set aside.
Heat oil in a large nonstick skillet over medium heat. Add onion, and cook, stirring constantly, 5 minutes. Add spinach, and cook, stirring constantly, 3 minutes.
Combine spinach mixture, Romano cheese, and next 7 ingredients in a large bowl; stir well. Spoon about 1 heaping tablespoon spinach-cheese mixture into each cooked shell. Combine pasta sauce and tomato sauce; stir well. Spoon 1/2 cup sauce into a 13- x 9-inch baking dish. Arrange stuffed shells over sauce in baking dish; spoon remaining sauce over shells. Cover; bake at 350° for 35 minutes. Uncover; bake an additional 10 minutes. Let stand 5 minutes before serving. Garnish with basil, if desired.