Pour 5 ounces champagne or sparkling wine into a glass, then stir in the 2 teaspoons rhubarb syrup.
Notes
Make a quick syrup. Use the recipe linked in the recipe card, or: place 1 cup chopped rhubarb, 1 cup sugar, and 1 cup water in a pot. Bring to a boil, then simmer until the sugar is dissolved and the rhubarb is soft, about 10 minutes. Then strain through a fine-mesh strainer to remove the syrup.
Add more syrup. I have "or to taste" in the recipe notes because it really is up to you. You can start with 2 teaspoons of rhubarb syrup, then add more if you want a darker and more strongly flavored drink. Alternatively, go lighter on the syrup for more of a "hint of rhubarb" experience.
Wine alternative. Try a non-alcoholic wine or flavored soda water (see tips below), ginger beer, or club soda.
Add a splash of alcohol. For a French 76 vibe, add a splash of vodka to the glass.
Mocktail: Easily make this cocktail into a mocktail by swapping sparkling water for the sparkling wine. You can use plain water or club soda, or try a flavored water. I'm partial to lime, strawberry, or watermelon, all of which pair beautifully with the rhubarb's unique flavor.