Peter Romaine at Penn State says simple button mushrooms can replace eggs in the production of many vaccines, and can replace chemical processes in the creation of other medicines as well.
The idea is to implant a gene which induces the production of a needed chemical into a mushroom's gills, kill all the mushrooms which lack the new chemical, then harvest the chemical after a few mushroom generations.
It's the rapid reproduction cycle of the mushroom which gives the technique its potency.
Of course, since you have a guy named Romaine replacing eggs with mushrooms, you don't really have an omelot at all.
You have salad.