Saute the onion in the butter until it is just beginning to soften, then add garlic and cook over medium heat for one or two minutes, do not let it brown. Stir in the flour and cook for one minute, then remove from heat and stir in the milk, slowly at first and then quicker as it all mixes together. I like to use a whisk for this. Place back over medium heat and stir until thickened. Add chili flakes and salt to taste, and then stir in prawns and heat until hot. Pour over cooked basmati rice. Sprinkle with chopped parsley