Place the 2 cups strawberries and sugar in a blender or food processor. Blend on high until the strawberries are liquified. Pour the puree into a fine mesh sieve with a bowl underneath. Use a spatula to help push the strawberries through the sieve. Discard the strawberry seeds in the sieve and refrigerate the remaining puree until ready to use.
When ready to make the Rossinis, pour about two tablespoons of the strawberry puree into a champagne flute. Fill up the the remaining glass with Prosecco, stir, and garnish with a strawberry.
You can make the strawberry puree up to two days in advance. Just cover and refrigerate until ready to use.
Add more or less strawberry puree depending on taste preference.
The Prosecco will bubble up quite a bit when added to the strawberry puree, so go slow when pouring. It helps to angle the glass and pour the Prosecco down the side of the glass rather than directly on top of the puree.
Prosecco is the classic drink to serve with Rossini cocktails. You can always substitute champagne or sparkling wine.
Use leftover strawberry puree in smoothies or for topping ice cream. It can also be stirred into sparkling water.