1. Make a thyme simple syrup by adding thyme sprigs, 1/2 cup sugar and 1/2 cup water to a saucepan. Bring to a boil over low heat and stir until the sugar dissolves. Remove from the heat and set aside.
2. Peel and chop 1 1/2 plums and muddle with the simple syrup and lemon juice.
3. Shake with ice in a cocktail shaker and pour an even amount over crushed ice in glasses.
4. Top off each glass with sparkling wine and garnish with extra plum slices and thyme sprigs.