Preheat & Prep. To start, preheat the oven to 375 degrees F. Set out a glass or ceramic 9x13 casserole dish.
Start The Green Beans. Bring a large saucepan of water to a boil, then add green beans and cook 5-7 min or until tender with just a tiny bite left. While the beans cook, prepare an ice bath by filling a large bowl of water with ice and cold water. When the beans are finished cooking, drain them, then quickly add to the ice bath while you finish the mushroom sauce. This will stop the cooking process and help preserve their vibrant color.
Start The Mushroom Sauce. In a large skillet, melt butter over medium heat. Add the mushrooms and cook 8-10 min, or until golden brown and tender.
Build Flavor. Add minced garlic, salt, pepper, and gluten-free flour and cook 1-2 minutes until fragrant.
Thicken & Finish The Sauce. Pour chicken broth and gluten-free tamari into the skillet and stir, making sure to scrape up any browned bits from the bottom of the pan. Cook 3-5 minutes over medium heat until thickened. Turn off the heat and stir in cream and parmesan cheese. Stir until cheese is melted and the sauce is evenly blended.
Combine. Remove the green beans from the ice bath and drain well. In a large bowl (or your skillet if you have room!), combine mushroom sauce and green beans and stir to coat evenly.
Layer. Pour green bean mixture into your prepared pan and smooth into an even layer. Sprinkle the stop of the casserole with crispy onions.
Cover + Bake. Tightly cover the pan with aluminum foil and bake 20-25 minutes, or until the sauce is bubbling around the edges of the pan. Remove the foil then bake and additional 5 minutes to toast the onions. Serve warm and store leftovers in an airtight container in the fridge 3-4 days.