Using a spoon or butter knife, vigorously stir the jelly in its jar. Measure out 2 tablespoons (1 ounce/shot) and add to a cup you can stir in. Add gin, sparkling water, and lemon juice. Stir until combined, then strain through a sieve into a 6-ounce glass filled with ice. Garnish with a slice of lemon and a sprig of basil.