In a cocktail shaker, muddle the strawberries with the lemon juice and sugar until the sugar dissolves into the lemon juice and the strawberries start to break down into a chunky paste. Add the vodka and elderflower liqueur, and stir together.
Spoon the contents of the cocktail shaker into two Collins glasses, add ice and top off with club soda. Stir. Garnish with a few strawberry slices.