Heat the grill/broiler to high.

Make the sauce: melt the margarine in a small saucepan, sprinkle in the flour and stir over a low heat for 1 - 2 minutes until it is straw-coloured. Remove from the heat and gradually stir in the milk. Return to the heat and simmer, stirring, until thick and smooth. Stir in 50 g (2 oz) of the cheese and the mustard and season to taste with salt and pepper. Stir until the cheese has melted, then remove the pan from the heat.

Peel the avocados, cut in half and remove the stones. Cut lengthways into thin slices. Peel, quarter and core the apples. Cut into thin slices. Arrange the slices of avocado and apple in layers in 1 large or 4 individual shallow gratin dishes (see Serving ideas). Squeeze the

lemon juice over them immediately to prevent discoloration.

Pour the sauce over the avocado and apple. Mix together the remaining cheese and the breadcrumbs and sprinkle evenly over the top.

Place under the grill/broiler for about 5 minutes until golden brown and bubbling. Serve at once.