To make the simple syrup, combine sugar, water, strawberries and rhubarb in a small saucepan. Bring to a boil over medium-high heat, stirring to dissolve the sugar completely.
Once boiling, remove from heat. Cover and let steep 30 minutes.
Strain and store in an airtight container in the refrigerator for up to 1 week.
To make the cocktail, add Grand Marnier and simple syrup to a champagne glass. Stir to combine.
Top with champagne. Serve.