Place lime wedges, ginger root, and leaves from 1 stem of mint into a cocktail shaker. Add 1 tsp stevia. Muddle well until ginger breaks down a bit and juices are released from mint leaves and lime.
Pour 1.5 oz of rum over the mint mixture. Top with ice. Cover with shaker top and shake until shaker becomes frosty.
Strain the contents of the shaker and pour over a highball glass filled with ice. Top with sparkling grapefruit water. Garnish with ginger root, lime wedge, and reserved mint leaves.