I agree. I had ruled it out previously because she went to the inexplicable effort to make individual Beef Wellingtons, but it really could have been in the satchel of gravy. Mixed in, heated, poured. Pretty simple. Especially after reading Erin's comment to her group that the volume of the mushroom is 90% less when dried (if that is true). It would make it much much easier to get the 20gms (ish) of mushroom in per guest.
Also very easy to rinse down the sink, or flush down the toilet, and even burn the packet and then you don't have any poisoned leftovers anywhere.
However I doubt she would have risked doing it in front of the guests, in case a guest said "no gravy thank you". I think she may have had it already on or IN each "pie" to ensure it got to its intended target
That or, she only poisoned half of the gravy, or she had two satchels of it.
The thing is, she apparently seemed odd about people going into her butlers pantry. Did she have the oven in there? A microwave? A dishwasher? I wish we had more detail on what was in the kitchen and what was in the butlers pantry. Also, they nearly always have sinks in them - maybe that's where she prepared it all and that's why the kids never saw other plates.
I still think that the death caps were strategically placed inside each individual Beef Wellington, but agree that it's possible it was actually in the gravy.