Making the orange juice mixture: Put the sugar, cloves, cardamom, bay leave, star anise, lemongrass and 1/2 cup water in a pot and bring to a boil. Reduce the heat and simmer until the spices release their flavor, about 5 minutes. Remove from the heat and cool for another 5 minutes. Combine the spice mixture and orange juice in a bowl, and refrigerate for 30 minutes. Once chilled, strain and discard the spices.
Take 4 champagne flutes. Put 4 lemon balm leaves into each glass and bruise the leaves with a muddler. Pour 1/2 cup juice and 1/2 cup prosecco into each glass. Decorate with fresh orange slices.