Preheat the oven to 375 degrees. Spray a 13 x 9-inch baking dish with cooking spray. Set aside.
Fill a shallow pan with hot tap water. Soak the lasagna noodles until pliable while preparing the filling. When pliable, drain and pat dry with paper towels.
Place spinach in a microwave-safe container and add 1/4 water. Microwave on high for 2-2 1/2 minutes or until wilted. Drain, cool, squeeze out any excess water and coarsely chop. Season the spinach with salt and black pepper to taste.
Combine spinach, chicken, artichoke hearts, cream cheese, scallions, garlic, 3/4 cup Mozzarella cheese, 1/2 cup Parmesan and salt and pepper to taste in a bowl. Set aside.
Heat butter over medium-high heat. Add flour and cook for 2 minutes, whisking constantly, being careful not to brown it.
Slowly add milk, whisking constantly. Bring to a boil, reduce heat to low and simmer until thickened. Stir in salt and pepper, 1/4 cup grated Parmesan cheese and a pinch of nutmeg.
Pour 1 cup of the sauce on the bottom of the prepared baking dish.
Place even amounts of the chicken filling on one of the short sides of the lasagna noodles. Roll up and arrange in a row on the bechamel sauce.
Pour remaining sauce over the cannelloni. Sprinkle remaining Mozzarella and Parmesan over the top.
Cover loosely with aluminum foil. Bake for 25-30 minutes. Remove foil and bake for another 25-30 minutes or until top is lightly browned and sauce is bubbling.