Add cranberries, juice, zest and sugar to a large pan over medium heat until berries break-down slightly and liquid is syrupy, 12-15 minutes.
Pour into blender and process. Be careful liquid is hot. Use a towel over the top to keep liquid from spraying because the heated liquid expands. Do not put top of blender on all the way, leave a space for the heat to expand.
Strain and discard any solids. Let liquid cool.
Pour 2 T. of the syrup into bottoms of 4 chilled champagne glasses.
Add a few cranberries to each glass and gently top off with chilled champagne.
Recipe by Geaux Ask Alice! at https://geauxaskalice.com/cranberry-kir-royale/