Place the raspberries in a small bowl with the sugar and muddle to release the juices and dissolve the sugar. Add the lemon juice to help the sugar dissolve - do a quick taste to see it is sweet enough (bearing in mind how sweet your limoncello is). If not, add a little more sugar. If the sugar doesn't dissolve, warm the mixture gently until it does then leave to cool.
Press the mixture through a strainer, holding back the seeds but getting as much liquid out as possible. Add the limoncello to the raspberry mixture, stir gently then pour into the bottom of a champagne flute.
Top up the glass with champagne, stir slightly to mix then serve.
Notes
Note you can make the raspberry puree (with or without the lemon) ahead of time and chill in the fridge a day or two. You can also make it in larger quantities for multiple drinks.