Via http://www.seriouseats.com/recipes/2011/12/23-skiddoo-champagne-... :

1 ounce gin
1/2 ounce St Germain Elderflower Liqueur
1/4 ounce thyme syrup
1/4 ounce fresh lemon juice
3 ounces chilled sparkling wine
Garnish: thyme sprig

Fill mixing glass with ice, add gin, St Germain, thyme syrup, and lemon juice. Stir until cold. Strain into flute, top with sparkling wine and garnish with thyme sprig.

For the syrup: Bring sugar and water to a boil in a small saucepan, stirring until sugar dissolves. Remove from heat, add thyme leaves, cover and let stand 1 hour. Strain and refrigerate.

1 cup sugar
1 cup water
2 tablespoons fresh lemon thyme leaves (or regular fresh thyme leaves)