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 for 2 minutes. Using a slotted spoon, 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 in 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 1 tablespoon garlic and sliced onion. 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. 

Make Sauce: Add remaining 4 tablespoons of butter to pan and let melt. Add 1/4 cup flour and mix to combine. Slowly add in 3/4 cup chicken broth and then 1 1/2 cups of creamer, 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 in 1 teaspoon salt, 3/4 teaspoon pepper, and 1 1/2 cups shredded cheese. Stir to combine. Taste and adjust salt or pepper, as needed.

Combine: Pour the creamy sauce over the green beans in the baking dish and add about 3/4 cup of the french fried onions; stir to combine.

Bake: Place in oven, middle rack and uncovered, for 20-30 minutes or until bubbly. Remove and cover with remaining cheese and french fried onions. Return to oven for an additional 5-10 minutes.

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