Could I substitute the cream with coconut milk - the fullfat, unsweetened kind? I'm trying to figure out how to make this parve for a kosher Thank...

Good luck.
If the altered recipe doesn't work out, look for (tested) parve green bean casseroles from Tori Avey, Jamie Geller or Noreen Gilletz.

Hi Michael! A couple other people have wondered about dairy-free alternatives, too. Copying my reply from the article page below. Hope it helps!

I haven't tested this recipe with a non-dairy milk or cream, so I can't say for sure—and I wouldn't want you to find out that it doesn't work on Thanksgiving. Your best bet would be a mini test run before the holiday: Sauté 10 ounces sliced mushrooms, deglaze with 1 1/2 cups coconut milk, and reduce until thick. Another option to consider: Make an ultra-thick cashew cream (https://food52.com/recipes...) and stir that into the sautéed mushrooms; you would just have to warm it up, versus reduce, since you can control the thickness when making the cashew cream.