Put the grenadine, lemon juice and cognac in a cocktail shaker with some ice.
Cover and shake (or stir well) to cool the mixture then pour into a champagne flute, straining to hold back the ice.
Carefully top up the glass with champagne.
Garnish with some pomegranate seeds on a cocktail stick, if you like (or you can use the traditional French 75 garnish, a lemon twist) then serve.
Notes
I recommend making your own grenadine, if you can, for a much better flavor. Otherwise, try to find as natural a bought version as you can. You can use a little less for a less sweet version, and depending on how sweet your grenadine is.You can also make this for a crowd - a bottle of champagne will make around 12 glasses, so you would need 6oz/180ml grenadine, 6oz/180ml lemon juice and 12oz/360ml cognac (or gin) for the base, then stir well with ice, strain into 12 glasses then top each up with champagne. You can also make the base mix ahead without ice and simply chill in the fridge for a few hours.