Squeeze the orange, grapefruit and lemon and mix the juice with the lillet.
Fill two glasses (Burgundy or Fancy) with ice cubes, add the juice / lillet mix and top up with sparkling wine (ideally a crémant, also a sparkling wine, cava or champagne - the main thing is traditional bottle fermentation and a cuvée).
As a decoration, you can add a slice of orange and / or grapefruit. Also very nice is a small chopper or an umbrella with a physalis skewered between a slice of orange!