1. Combine first 3 ingredients and 1 1/4 cups water in a medium saucepan. Bring to a boil over high heat, stirring occasionally. Reduce heat to medium, and simmer 5 minutes.
2. Pour mixture through a fine wire-mesh strainer into a bowl, using a spatula to squeeze out juice. Discard fruit pulp and seeds. Cool mixture completely; transfer to an airtight container, and chill up to 2 weeks.
3. To serve, divide 1/4 cup rhubarb mixture among Champagne flutes or wineglasses. Top with about 1/2 cup sparkling wine. Garnish, if desired.