Separate the butter into 3 equal parts. Melt 1 tablespoon of the butter and combine it gently with the breadcrumbs and parmesan cheese. Set the mixture aside.
In an ovenproof skillet (8-10 inches), heat 1 tablespoon of the butter with the garlic over medium heat. When the butter melts, cook the garlic for 1 minute, then add the spinach with a sprinkle of salt. Stir the spinach to wilt for 1-2 minutes, then stir in the parsley and remove the mixture to a plate to rest. Turn the heat off the skillet and add the last tablespoon of butter.
When the butter melts and coats the bottom of the skillet, add the mushrooms in one even layer. Spoon the spinach mixture on top and sprinkle the breadcrumbs over the whole skillet.
Bake at 400F for 10-15 minutes, until the mushrooms are wilted and the breadcrumbs have browned. Serve warm as a side dish or over bread as crostini. Enjoy!
Notes
Prep: You can make this entire dish in advance up to the last baking step (skip it for now), then cover the skillet and keep it in the fridge for up to 3 days. To serve, bake the dish as instructed, adding 5-10 minutes to the baking time to be sure the breadcrumbs get nice and crispy.Oyster Mushroom Variation: This recipe can be made with any mushrooms you like, just watch the cook time to be sure denser/thicker mushrooms have time to cook through.Gluten Free: Make this gluten free by using GF breadcrumbs in place of the panko.