To make rhubarb syrup bring 1/2 cup water, 2 chopped stalks of rhubarb and a dollop of honey to boil. Reduce heat and simmer for 10-15 minutes or until rhubarb softens and releases juices. Pour into a blender and blend until smooth. Set aside in a bowl to cool. Note, this is not really a syrup but more like a mousse consistency.
Dip glasses in lime juice and salt. Set aside.
Combine all ingredients into a blender and blast on high until smooth.
Pour into prepared glasses and garnish with a rhubarb stalk or rosemary sprig (or both!)
Pour green chartreuse into a cocktail glass. Add rosemary sprig and set aside. Shake the remaining ingredients with ice. Carefully light the green chartreuse, allowing the rosemary to boil a few seconds to release the oils. Extinguish with the remaining ingredients. Top with crushed ice.