To begin preheat your oven to 375 degrees. Then trim the green beans & slice the mushrooms.
Bring a pot of water + 3 tsp salt to a boil. When water is ready, add green beans. Blanch for 5 minutes.
Immediately drain beans when done & place them into bowl filled with ice water. Allow to cool for 5 minutes then drain and set aside until needed.
Into a large skillet, melt 2 tbsp of butter over medium heat.
Next add mushrooms and sauté for 3-4 minutes until begin to brown. Add garlic, onion powder, salt & pepper. Continue cooking for 1 minute until fragrant.
Add chicken broth to skillet and allow to simmer for 2 minutes.
Finally add cream of mushroom to the skillet and simmer for 5 minutes.
When sauce is ready, add in your green beans. Stir all ingredients together.
If using an oven safe skillet: sprinkle fried onions over top and cover dish with foil.
If using a separate dish: place casserole ingredients inside the dish, sprinkle fried onions over top and cover dish with foil.
Bake casserole in oven for 15 minutes. After 15 minutes, remove the foil and allow casserole to continue cooking for 5 more minutes.
When dish is finished cooking, remove from oven. Allow to cool for 10 minutes, serve & enjoy!