Oven: Preheat oven to 350 degrees F.

Blanch Green Beans: Fill a medium saucepan 3/4 full of water and bring to a simmer. Meanwhile, take a large bowl and fill 3/4 of the way with water. Add ice to create an ice bath. Set aside. Add green beans to the simmering water. Let cook 2-4 minutes. Using a pair of tongs, carefully remove the green beans from the simmering water and place them directly into the ice bath. Let sit for 5 minutes or until fully cooled. Remove and place into a 9x13 baking pan.

Cook Onions + Mushrooms: Bring cast iron skillet to medium-high heat. Add 3 tablespoons of butter and let melt; swirl to coat the pan. Add onion and garlic. Move around pan until until the onions have softened, approximately 2-3 minutes. Add mushrooms and move around pan until they have softened, another 2-3 minutes. Remove mushrooms and onions with slotted spoon and place on a plate. Set aside.

Make Sauce: Add remaining 4 tablespoons of butter to pan and let melt. Add gluten free flour and use a whisk to combine into a paste (a.k.a. a roux). Slowly add in the creamer and then the chicken broth, whisking the entire time and ensuring the mixture is hot enough to just barely simmer. Let simmer, uncovered, 3-5 minutes. Once thickened remove from heat and add back in the mushrooms and onions as well as the salt, pepper and red pepper flakes (if using). Stir to combine. Taste and adjust salt or pepper, as needed.

Combine: Pour the mushroom mixture over the green beans and add approx. 3/4 cup of the gluten free french fried onions; stir to combine.

Bake: Place in oven, middle rack and uncovered, for 20-25 minutes. Remove and cover with remaining gluten-free french fried onions. Return to oven for an additional 5-10 minutes.

Cook + Serve: Remove and let cool sightly before serving (sauce will thicken as it cools).