Begin by melting the butter in a medium saucepan, add the flour and blend to a smooth paste.
Cook for 2 minutes, then gradually stir in the milk, stock and cream and, stirring all the time, bring it to simmering point and cook very gently for 2 or 3 minutes. Then remove the pan from the heat, add the chicken pieces, sherry, salt, pepper and lemon juice to taste and stir everything together until combined. Now halve and quarter the avocados and, having removed the stone and skins, slice the flesh thinly and cover the base of the dish with the slices.
Sprinkle over a little lemon juice, spoon the chicken mixture on top and, finally, add the grated cheese. Transfer to the pre-heated oven and bake for 20-25 minutes or until the sides start to bubble.
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