In a medium saucepan, combine rhubarb, ½ cup water, and sugar. Bring to a boil over medium-high heat; reduce heat and simmer for 10 minutes. Remove from heat and let cool.
Stir in lime juice. Strain mixture through a fine-mesh sieve, discarding solids. Refrigerate to chill.
For each serving, pour approximately 2 tablespoons syrup over ice, and fill glass with sparkling water. Garnish with rhubarb ribbons and lime slices, if desired.
Notes
Note: Do not stir after mixture has come to a boil to avoid breaking up the rhubarb too much and to keep the syrup clear.
Recipe by Victoria at https://victoriamag.com/rhubarb-key-lime-fizz-recipe/