I followed Dave Arnold's advice for coriander syrup, which proves to be an interesting replacement for ginger syrup. It's a neat substitution in one of my favorite highballs.
2 oz blended scotch (Famous Grouse this time)
1 oz coriander syrup*
3/4 oz lime juice
3-4 oz soda water (to taste)
Build over ice in a tall glass. Large ice cubes are recommended.
*This is a simplified version of a recipe from Liquid Intelligence, but it's still a bit more involved than my typical syrup method, so I figure I should explain. Combine 1 1/2 cups sugar, 1 1/2 cups water, 1/2 cup of coriander seeds, 1 teaspoon black peppercorns, and 1-2 whole dried arbol chilies in a saucepan over medium heat. Bring the whole thing up to a simmer until the sugar dissolves and then blend to break up the solids. You can do this in an actual blender (and I would have with a larger batch) or very carefully apply a hand blender. Remove from the heat and let the whole thing infuse until you're happy with the flavor, at which point run it through a strainer and a coffee filter while it's still warm.
I was terrified that this name would turn out to be a character from 90210 or something, but he's actually a heavy metal guy. Much more appropriate!