Strain puree through a fine mesh strainer to remove seeds.
In a cocktail shaker, combine strawberry puree, lemonade, and simple syrup.
Add ice and shake gently for 10–15 seconds.
Fill each glass halfway with strawberry lemonade mixture.
Top with chilled champagne.
Garnish with a lemon slice and mint leaf.
Notes
Chill champagne before serving to keep drinks cold without excess ice. Adjust strawberry-to-lemonade ratio for desired flavor. Strawberry lemonade base can be made a day in advance.