Cook broccoli according to pkg directions, but omit salt and add onion. In a large saucepan over medium high heat, melt butter, cook 1 minute. Add the 2 tbsp flour, whisk 1 min. Slowly whisk in milk and salt until smooth and boiling. Add broccoli and grated cheese, continue to whisk. Bring to a boil. Season with cayenne and nutmeg if desired.