Place some ice in a rocks glass or similar sized glass.
In a cocktail shaker, muddle the mint leaves, rosemary sprig, lemon slice, and sugar together until the mint is very aromatic and slightly bruised looking and the sugar is mostly dissolved.
Add the lemon juice and shrub and top with ice.
Shake well, until the shaker is very cold and is sweating.
Strain into the prepared glass and top with soda water. You can garnish with a lemon wedge or slice, mint leaves, rosemary, and edible flowers if desired.