Add the contents of the jelly packet to a large heatproof bowl, pour 1 cup of boiling water over the jelly crystals, and stir with a fork until the jelly crystals have dissolved.
Chill the jelly in the refrigerator until it is the consistency of unbeaten egg whites.
When the jelly is partially set, add the 11/2-13/4 cups of well-chilled evaporated milk, the orange zest and the Cointreau. Basically add 11/2-13/4 cups of liquid, so 11/2-13/4 cups of evaporated milk if you are NOT adding Cointreau. And if you are adding Cointreau, reduce the quantity of evaporated milk by the amount of Cointreau added.
Beat the mixture with an electric beater until it is light and fluffy, fold in the cream and most of the strawberry slices. Reserve some strawberry slices for garnish.
Spoon the mixture evenly into 8 parfait glasses or dessert dishes. If you use smaller glasses, you may well have enough for 10 serves. It looks more elegant in parfait glasses.
Chill the mousse for several hours, preferably overnight.
Just before serving, dust each parfait glass with sifted icing sugar (optional), top with several slices of strawberry and serve immediately.
Chef's Note: Fruits vary enormously in how much moisture they contain, so when it comes to deciding how much liquid to add (i.e. the evaporated milk and the Cointreau) be mindful of this. Since posting this recipe, having not used this recipe for a while, I made this mousse and for the first time added black cherries. The mousse still tasted delicious, but the cherries being a lot heavier than strawberries sunk to the bottom - not a major problem; I still made sure that the mousse and cherries were evenly distributed between the parfait glasses. But because the cherries had a lot more moisture in them than do strawberries, the consistency of the mousse was not as thick as I should have liked. So I have amended the total quantity of liquid to 11/2-13/4 cups Originally, I had specified TWO cups of liquid. It's probably better to err on the side of adding a little less liquid - but it really does depend a lot on what fruit you are adding.