In a large saucepan, cook sausage and onion until meat is no longer pink; drain. Stir in the water, lentils, bouillon and red pepper flakes. bring to a boil. Reduce heat; cover and simmer for 25-30 minutes or until lentils are tender. Stir in spinach, cook 3-5 minutes longer or until spinach is tender. Sprinkle with cheese.