Preheat your oven to 400°
Heat a cast iron skillet over medium heat.
Chop the mushrooms. (fine dice for those who don't love mushrooms, rough chop if you want to taste the mushroom texture.)
Saute for 2-3 minutes until the mushrooms release their liquid.
Sprinkle the flour from the onions over the mushrooms to create a roux.
Cook the roux for another 3-4 minutes until the flour is toasted.
Next, add the broth slowly, stirring to prevent clumps.
Simmer the chicken broth until thickened, about 4-5 minutes. You'll know its thickened when your spoon leaves a trail showing the skillet behind it in the broth.
Add the cream and stir to combine.
Simmer another 4-5 minutes until, again, the sauce is heated through and thickened.
Reduce the heat to medium low.
Add the shredded gouda cheese!
Once the cheese is melted and fully incorporated, it's time to add the green beans.
Gently fold in the green beans.
Pour your prepared green bean casserole into a casserole dish.
Top your green beans with the fried onions.
Bake the assembled casserole for 10 minutes.