Place a handful or two of ice in a highball 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 gin and shrub and top with ice.
Shake well, until the shaker is very cold and is sweating.
Strain into the prepared glass, top with club soda and garnish. You can garnish with a lemon wedge or slice, mint leaves, rosemary, and edible flowers if desired.