Puree the strawberries, sugar, liqueur and lemon juice in a blender until very smooth (makes appoximately 1½ cups puree).
Pour through a fine strainer set over a container with a pouring spout.
Pour the puree into glass flutes to fill by ⅓ (about 2-3 tablespoons per glass). Slowly pour Prosecco over the puree and stir gently — it will bubble up. Garnish glass with a slice of strawberry and enjoy.