Preface: The only other context I’ve eaten matsutake is in dashi and in a quesadilla (a delicious quesadilla). I don’t think I would make this dish exactly the same next time; I think it would be better in more broth, without the creme fraiche, with egg noodles (Asian style). I used really good quality, fluffy, fresh gnocci for this dish and I think it was really competing with the matsutake, which is a shame because they should have been the feature of the dish.
Luckily, matsutakes had a bumper crop this year and they were only $20/lb. In SF, they can be found for $40 or more most years, so this was a really nice surprise last time I popped in at the Japanese market. Yesterday, I even saw them at Rainbow Foods!
I do think simmering the matsutakes in dashi (it was small amounts of mirin, soy sauce, sake, walnut oil and then a larger amount of straight up dashi–water infused with kombu and i-forget-the-name tuna flakes) worked well, I just would have made more of it next time and omitted the creme fraiche I added. The watercress also worked well. So go ahead and do that, and sub those gnocci for egg noodles and I’m pretty sure you’ll have a delicious meal.
Anyway, no recipe here since I won’t stand behind it, but thought you might learn from my experience.