Make cashew cream: In a medium bowl, pour boiling water over cashews and let sit until they soften, at least 15 minutes and up to 30. Stir in yeast. Puree mixture in a blender on highest setting until smooth and creamy, about 3 minutes. Season
with salt to taste. Set aside.
Make breadcrumbs: Pulse bread in a food processor until coarsely ground. Set aside. (You should have 1 cup.)
Assemble gratin: Preheat oven to 375 degrees. Cover bottom of an 8-inch-round baking dish with a single layer of rutabaga slices, overlapping edges and working in a circle. Season with salt, pepper, and
some of the chopped herbs. Add another layer and season with salt, pepper, and herbs. Pour in about 1/3 of cashew cream -- enough to cover both layers. Continue until baking dish is full. Pour in remaining
cashew cream. Sprinkle nutmeg over top layer. Toss breadcrumbs with oil in a small bowl. Top gratin with breadcrumbs.
Bake on a rimmed baking sheet until rutabagas are tender when pierced with a sharp knife and breadcrumbs are golden brown, about 1 hour and 15 minutes.