Put the onions and herbs in a pan with the milk and bring to the boil. Remove from the heat and leave the milk to infuse for 10 minutes. Strain.
Melt the butter in a pan and stir in the flour to make a roux. Cook gently, without colouring, for 5 minutes. Remove from the heat and gradually mix in the strained milk. Return to the heat and bring to the boil, stirring constantly to make a smooth sauce. Season to taste with salt and cayenne pepper. Simmer very gently, stirring, for 5 minutes. Taste and adjust seasonings.
© 1990 Joyce Molyneux. All rights reserved.