Add the sugar, water, rhubarb, fennel and orange zest to a medium saucepan. Cook over medium-low heat, stirring occasionally, until syrup thickens slightly and rhubarb starts to soften and break down (~15 minutes).
Strain through a fine mesh strainer. Use the back of a spoon to push on the rhubarb and fennel to extract any remaining liquid from the pulp. Let cool and store in the refrigerator until ready to use.
For the cocktail
Add the lemon juice, rhubarb fennel syrup, gin, salt, and vermouth to a shaker filled with ice. Shake and strain over new ice into an old fashioned glass. Garnish with a fennel frond and a rhubarb ribbon. (note: a vegetable peeler works well for making nice long rhubarb ribbons).