Put the strawberries, sugar and lemon juice in a blender and puree until smooth. Taste for sweetness, and add more sugar if you like.
Once it tastes right, you can pass the puree through a fine sieve to get rid of the strawberry seeds if you like (I don't, because I enjoy their crunch).
If you're making strawberry juice, stir 3 tablespoons of the puree into about 5 ounces of cold water. Add ice if you like.
To make a Rossini, put 3 tablespoons of the strawberry puree in a tumbler or a large wine glass. Pour a little Champagne into the glass, and combine with the puree by stirring gently with a spoon. Fill the glass with Champagne (it will foam up as you pour -- this is part of the fun) and serve immediately, preferably with some salty little cheese biscuits.