I have seen pot roast of Shitake. Pot roast = over-cooked obviously.
There were pot roast for: duck roast, chicken roast, pork trotters, buddhist faux meat, Buddha jumped over the wall roast. All of them amply roasted with shitakes.
Okay, for what I have tried, I encountered two kinds of shitake pot roast
- sweet and sour
- sweet and salted soy-sauced
The Buddhist faux meat:
I saw the usual buddhist faux meat the permutations of which I think are due to diverse combination of various flours, soy and other beans. The thick gravy is dark brown with obvious scent of dark sweet soy sauce and black vinegar. And the over-cooked shitakes are chewy juicy with the wonderful gravy.
Chicken pot roast:
Then there was another time, the chicken pot roast. The gravy was thinner but not sour. It was a whole chicken that was boiled in sauce with the shitake. From what I understand, the shitake roast was done first and then the chicken submerged in the boiling cauldron at the end of the roast. At the last 10 minutes may be. Shitake juiced up with gravy enriched with chicken taste/flavour.
Chewing on the shitake
You should try overcooked shitake that has absorbed the juicy gravies. Chocolate mousse cake is nowhere in comparison just as you sink your teeth into it and the juice runs down your chin. And then you lick your fingers after you had them wipe your chin.
Slow cooked shitake pot roast. I am sure there were spices involved. I also believe they used shitakes that had been stabilised by drying (i.e. dried shitakes). Otherwise, I doubt fresh shitakes could take the strain of being overcooked.
In general, cold food needs stronger flavors to taste equally good. A little more seasoning, a little stronger spicing, a touch more acidity.
However, you also have to consider texture and cultural expectations.
I am not sure that cold cooked mushrooms are... expected. You might want to try it in advance and see what you think. On the other hand, raw mushrooms would be dissonant as well with stuffing.
I would suggest reassessing what you are taking. Perhaps a crudite, where your fillings transform into dips, and you supplement the (raw) mushrooms with other vegetables might be a logistically more appropriate dish.
If you want hot dips, they can probably be done in a microwave, if your office has one.
Best Answer
It's important to note that wash or no wash, mushrooms themselves already have a high water content so their ability to absorb more water is limited. Estimates I've found put the range between 75-90% water, depending on the type of mushroom. Cooks Illustrated, for example, puts it at 80%.
Sources such as Cooks Illustrated, Alton Brown, Harold McGee, Robert Wolke, and Kenji Lopez Alt have tested washing mushrooms by weighing them pre- and post-wash, and all found that mushrooms absorb a minimal amount of water that does not significantly affect cooking time. Both McGee and Wolke soaked the mushroom for five minutes - so much longer and with more direct contact than than a normal simple rinsing. Lopez Alt's testing, for example, found that the mushrooms absorbed only about 2% of their total weight which translated to an extra 15 to 30 seconds of cooking time. After testing, these sources all encourage washing mushrooms.
Several of the sources also observed that most of the additional weight is on the surface of the mushroom, which can either be patted dry with a towel or paper towel, or removed with a salad spinner, as other previous answers here have noted.
Cooks Illustrated suggests only washing whole mushrooms, as when cut they can become more absorbent. They also suggest that if you are serving the mushrooms raw, use a tooth brush to brush them clean rather than rinsing because rising can cause discoloration.
Washing should be only done just before cooking. Washing and then storing mushrooms can shorten their shelf life and they can become slimy on the surface.
Wolke notes that if your mushrooms are steaming rather than browning, it's more likely that your pan is too crowded rather than a function of having washed them.
Cooks Illustrated describes their process here, McGee in The Curious Cook, Wolke in What Einstein Told His Cook, Lopez Alt in The Food Lab (and also Serious Eats) and the best I could find for Alton Brown were transcripts from the Good Eats Fan Page. Both the the Cooks Illustrated and Serious Eats links also have more general tips on mushroom storage and prep.