For the Winter Citrus Cordial (Citrus Syrup):

Using a peeler, remove the zest from the citrus in wide strips, trying to avoid the bitter white pith as much as possible. Use a paring knife to slice or scrape off any excess pith from each strip.

Juice the citrus, then weigh the juice on a kitchen scale. Add the juice and an equal amount (by weight) of sugar to a blender, and blend on high speed until the sugar dissolves.

If using the sous vide method, carefully transfer the peels and juice/sugar mixture to a plastic bag, and seal according to sous vide instructions. Submerge in a 135 degree Fahrenheit water bath for 2 hours, then plunge bag into an ice bath to cool. Strain before use and store in the fridge.

If using the overnight method, combine the peels and juice/sugar mixture in a tightly sealed container, and let sit in the fridge overnight. Strain out the peels before use, then store in the fridge.

For the Champagne Citrus Cordial:

Add the citrus cordial to the bottom of a chilled champagne flute. Top with chilled champagne. Garnish with citrus peel.