Fill a large pot with lightly salted water and bring to a rolling boil. Stir in manicotti and return to a boil. Cook pasta uncovered, stirring occasionally, until barely tender yet firm to the bite, about 5 minutes. Drain carefully and rinse with cool water.

Preheat the oven to 350 degrees F (175 degrees C). Coat a 9x13-inch baking pan with cooking spray.

Cook chicken and onion in a medium-sized frying pan until chicken is no longer pink and broken into small pieces, 7 to 10 minutes. Add garlic and cook 1 minute. Add spinach, cream cheese, artichoke hearts, and 6 ounces of Italian cheese blend. Mix well and set aside.

Combine milk, cottage cheese, cornstarch, salt, pepper, and nutmeg in a food processor to make a smooth sauce. Pour sauce into a saucepan, add Parmesan cheese, and heat over medium heat until cheese has melted and sauce has thickened, about 5 minutes.

Pour enough sauce into the bottom of the prepared pan to coat the bottom. Stuff each cooked manicotti shell with chicken mixture and place into the pan on top of sauce. Pour remaining sauce over stuffed manicotti in the pan and top with remaining 2 ounces Italian cheese blend. Cover the pan with a lid or aluminum foil.

Bake in the preheated oven until heated through, about 30 minutes.