To make the raspberry mint simple syrup, combine sugar, water, mint leaves and raspberries in a small saucepan.
Bring mixture to a boil, reduce heat and simmer for 5 minutes, stirring occasionally until the sugar is dissolved.
Remove from heat, cover and let cool for 20 minutes. Strain before using.
To make the champagne sangria, combine champagne, orange juice, raspberry mint simple syrup, raspberries, and strawberries.
Serve immediately in coupe glasses, making sure to get the fruit in the glasses. Garnish with a fresh mint sprig.