Basically a French 75 variant using some of the ingredients on hand. We tried a couple different versions with one citrus or another before splitting the difference.
2 oz London Dry gin (really any gin would do fine here; cheap gin would be suitably authentic)
3/4 oz rhubarb syrup (the same lightly-spiced version from N.C. #6)
3/4 oz lime juice
3/4 oz grapefruit juice (I used a nice sweet Ruby Red variety)
Shake and strain over fresh ice, then top with about 2 oz rose cava or champagne (I think we actually used a Baron de Seillac Brut Rose, which is from Provence and therefore technically neither, but that's irrelevant; anything pink and bubbly and dry will do fine).