Step 1Clean the mushrooms and cut them into one-eighth-inch slices. In a medium skillet, heat the butter over high heat just until melted, then add the mushrooms and saute them until golden, about 10 minutes. Drain any liquid from the mushrooms and set them aside.
Step 2Place 2 quarts of water in a medium stock pot with one-half teaspoon salt. Bring the water to a boil over high heat, then stir in the spinach and blanch for just a few seconds, until wilted and bright green in color. Remove the spinach using a slotted spoon to a large bowl of ice water until cool. Drain the spinach well, squeezing out any excess water.
Step 3Heat the oven to 350 degrees. Reduce the cream to one-half cup in a small saucepan over low heat. Strain the cream into a small bowl and season with a pinch each of salt and pepper.
Step 4Butter a small (2-quart) baking dish, and spread the spinach in the bottom in an even layer. Sprinkle the top of the spinach with one-eighth teaspoon salt. Arrange the mushroom slices over the spinach in a fish-scale pattern. Spoon the cream evenly over the top and bake for 5 to 10 minutes, until heated through. Serve immediately.