In a cup, soften the gelatine in the water.
Place the cup in a pan of simmering water to dissolve the gelatine completely.
Set aside.
In a medium sized mixing bowl, beat the egg yolks, half the sugar, the salt and the lemon rind together with a wire whisk until they are pale and thick.
Gradually add the hot milk, stirring constantly.
Pour the mixture into a medium sized saucepan and set it over low heat.
Cook the custard gently, stirring constantly with a wooden spoon, for 6 to 8 minutes, or until it coats the back of the spoon.
Be careful not to boil the custard or it will curdle.
Remove the pan from the heat and pour the custard through a strainer back into the bowl.
Stir in the gelatine and the lemon essence.
Allow the custard to cool to room temperature, then place it in the refrigerator to chill for about 45 minutes or until it is beginning to set.
Meanwhile, in a small mixing bowl, beat the cream with a wire whisk or rotary beater until it is thick but not stiff.
Set aside.
In another bowl, beat the egg whites with a wire whisk or rotary beater until they are frothy.
Add the remaining sugar and continue beating until they form stiff peaks.
With a metal spoon, fold the whipped cream into the almost setting custard.
When it is completely mixed in, fold in the egg whites.
Pour the mixture into the pastry case.
Place the flan in the refrigerator and chill it for at least 2 hours, or until the filling is firm and very cold.
Sprinkle over the toasted almonds and serve.