To make roasted pear syrup, slice 4 Bartlett pears and mix with one tablespoon of sugar. Put the pears on a sheet pan and roast at 350 degrees F for about 15 minutes, or until the pears start to caramelize. Blend the pears and their juices in a blender and strain out the solids.
For the Pear and Elderflower Collins:
Shake the gin or vodka, pear syrup, St. Germain, and lemon juice with ice and strain into a Collins glass filled with ice. Top with sparkling water.