Warm the olive oil in a large skillet over medium heat. Once hot, add the mushrooms and season with a pinch of salt and pepper. Sauté until softened, then stir in the minced garlic and continue cooking until the liquid in the skillet has evaporated.

Incorporate the butter and flour into the skillet. Stir until the butter melts and forms a paste coating the bottom of the skillet. Keep stirring and cook for one to two minutes, or until the flour begins to turn golden brown.

Pour in the vegetable broth, milk, and garlic powder. Whisk everything together, making sure to dissolve any browned bits from the bottom of the skillet. Allow the mixture to simmer gently, which will cause it to thicken.

Add the cream cheese and whisk over medium heat until it completely melts into the sauce.

Gently fold in the frozen green beans, ensuring they are well-coated in the sauce. Continue heating over medium, stirring occasionally, until the green beans are warmed through and the sauce returns to a gentle simmer. Let the green beans simmer for a few minutes, then taste and adjust seasoning with salt and pepper (I used about ½ tsp salt and an additional ⅛ tsp pepper).

Stir in ¼ cup of crispy fried onions, then transfer the green bean mixture to a 2-quart casserole dish. Sprinkle the remaining ¾ cup of crispy fried onions over the top.

Bake the casserole for 20-25 minutes, or until the fried onions achieve a deep golden brown color and the sauce bubbles around the edges.