Place the sugar, water and hibiscus in a saucepan over medium-low heat. Whisk until the sugar dissolves, bringing the mixture to a simmer. Cook for 1 minute. Turn off the heat and set the saucepan aside. Let it cool completely. Strain the mixture through a fine mesh sieve to remove the leaves. Store in a sealed container in the fridge! The syrup will make about 3/4 cup.
Fill each glass with 1 to 2 tablespoons hibiscus syrup. Top it off with 4 to 6 ounces of rosé champagne and serve immediately! You can garnish with fresh herbs (like lavender!) if desired. I find that one bottle of champagne will make about 8 to 10 mimosas, and this is more than enough syrup for that! If you have leftover, stick it in the fridge for other cocktails.