Heat skillet, spray with cooking spray, add diced chicken and cook until no longer pink. Remove chicken. Spray pan and saute onion, mushrooms and garlic. Use 1-2 T of water if needed. Cook until onions are transparent then add back chicken and stir to combine. Remove from heat.
Combine soup and sour cream in a bowl. Spoon half of the mixture into another mixing bowl and add chicken and saute mixture. Stir until combined then stuff the manicotti shells with the chicken mixture. Place shells in a 13x9 baking dish coated with cooking spray. Spread reserved 1/2 soup mixture over top of shells. Sprinkle top with cheese.