To make the jelly, combine prosecco and lemon juice in a large bowl. In a separate bowl, soak gelatine leaves in cold water for 5 minutes to soften. Set bowls aside.
Warm elderflower cordial in a saucepan over medium heat for 3-4 minutes until small bubbles start to form and mixture is hot. Remove from heat and set aside to cool slightly. Squeeze excess water from gelatine, then stir into warm cordial until melted and combined.
Add gelatine mixture to prosecco and stir very gently with a spatula or large spoon until completely combined. Transfer bowl to the fridge, stirring every 15 minutes, for 1 hour until jelly is almost set but still liquid (this ensures the fruit does not float to the top when added).
Reserving 100ml jelly, pour one-quarter jelly into a 1L (4 cup) jelly mould. Stud with one-quarter fruit and mint. Repeat layers three more times.
Heat the reserved jelly until liquid, then pour over the top – this will create a flat base, so the jelly will sit flat. Cover with plastic wrap and chill overnight.
To serve, dip the base of the mould in warm water for 30 seconds. Gently invert the mould onto a serving platter. Keep chilled until ready to serve.
Decorate with flowers, if using, extra mint leaves and lemon zest.