Place water in a pot and add broccoli florets. Bring to a boil, cover and lower flame. Cook for 5 minutes until florets are soft.
In a seperate pot mix corn starch, salt, pepper, and milk. Cook for 1 minute until mixture is thick. Mix in onions and cheddar cheese, mix until cheese has melted.
Drain broccoli and add to cheese mixture.
Place parchment paper in a pan, and spray with cooking spray. Add cheese mixture and sprinkle parmesan. Cover and bake on 350the for 25-30the minutes, or until vegetables are soft.