Preheat the oven to Gas 6, 200 C. Grease the Pyrex souffle dish with a little butter and sprinkle the inside with the parmesan cheese, all the way up the sides (this helps the mixture to grip the sides when rising).
Cook the broccoli for 3-4 minutes in boiling water, drain well and put into a food processor to chop it finely.
Put the butter, flour and milk into a saucepan and cook over a medium heat, stirring continuously until thickened (it should be a thick sauce).
Remove from the heat, add the mustard and cheese and stir until melted. Add the egg yolks and broccoli and stir well.
Whisk the egg whites until stiff and then fold gently into the cheese mixture being careful not to knock too much air out. Pour into the soufflé dish.
Cook in the oven for 30-35 minutes until well risen and brown, do not open the door during cooking.
Serve immediately with crusty bread, (it will be a bit gooey in the centre still).