Cook pasta according to package directions, omitting salt and fat; set aside.
Coat a small nonstick skillet with cooking spray, and place over medium-high heat until hot. Add onion and garlic; sauté 3 minutes. Remove from heat; set aside.
Combine 1/2 cup mozzarella cheese, 1/4 cup Parmesan cheese, and next 5 ingredients (mozzarella through nonfat cream cheese) in a bowl; beat at medium speed of a mixer until smooth. Stir in onion mixture and spinach.
Spoon mozzarella cheese mixture into cooked manicotti (about 1/3 cup per shell).
Divide 1 cup sauce evenly between 6 individual casserole dishes coated with cooking spray. Arrange 2 stuffed manicotti in each dish. Pour remaining sauce over each serving. Place dishes on a baking sheet. Cover each dish with foil, and bake at 350° for 25 minutes. Sprinkle with remaining mozzarella and Parmesan cheeses; bake, uncovered, an additional 5 minutes. Garnish with oregano, if desired.