Definitely complete the lizards quest (The Curse of Lust) but you don't need to finish it (to completion) first to complete Bernard's sickness.
So long as you know, and have made Heat Occisia. You should find the Spectral Sirup in the potion making tab (the mortar and pestle) under learned. I'll put the recipe down as well.
1st: Pulp (Blue)
2nd: Heat Occisia
3rd: Pulp (Red)
4th: Heat Occisia
5th: Pulp (Blue)