Heat on medium and occasionally stir until the sugar dissolves.
Remove from the heat and let cool. Refrigerate for up to 4 weeks.
For the strawberry puree
Place strawberries, simple syrup, and lemon juice in a food processor.
Process for several seconds until pureed. Strain the mixture.
For the Rossini cocktail
Pour 2 tablespoons of strawberry puree into a chilled champagne glass.
Pour 4 oz Cava into the glass.
Garnish with a strawberry slice.
Notes
Adjust the amounts of the ingredients to your taste. Want more sweetness? Add more simple syrup. Want more tartness? Add more lemon juice. Less strong? Replace the Cava with sparkling water.