Preheat the oven to 350 F. Grease the inside of a 9x13 baking pan with butter or cooking spray and set aside.
In a large pot, add water (enough to submerge the green beans) and bring to boil. Once boiling, add in the green beans and boil for just 2-3 minutes. Transfer to a colander and drain out the water.
In the same pot over medium heat, add avocado oil and sauté the chopped onions and mushrooms for 3-5 minutes until tender and the onions start to get translucent. Add the minced garlic and sauté for 1-2 more minutes until fragrant.
Add the cream of mushroom soup, buttermilk, and chicken broth to the pot and mix to combine. Stir occasionally and cook for 5-8 minutes until heated through.
Remove the pot from the heat and stir in the green beans, shredded cheese, ¾ cup of the crispy fried onions and mix until well combined.
Pour the mixture into the prepared baking pan and bake uncovered for 30-35 minutes, until the sauce is bubbly. Remove the pan from the oven and top the casserole with the remaining 2 ¼ cups of crispy fried onions. Return to the oven and bake for an additional 10 minutes, just to get the onions hot and give it an extra crisp.
Garnish with fresh chopped parsley if desired and serve as a side. Enjoy!