Preheat oven to 350F degrees. Prepare a 13 x 9 casserole dish with cooking spray and set aside.
In a large saute pan, cook the mushrooms over medium-high heat until they are browned and soft, about 5-8 minutes. Lower heat to medium-low. Add garlic and cook for another minute.
In a bowl, whisk together almond milk, cornstarch, and onion powder. Pour into the pan with the mushrooms. Bring to a slight boil, then lower the heat again and simmer until slightly thickened, about 5 minutes. Don't worry if your milk starts to separate. The cream cheese will bring it all back together.
Slowly whisk in cream cheese until melted, then whisk in parmesan cheese, salt, and pepper. Continue to simmer until the sauce is quite thick, about 2-3 minutes.
If you like your green beans on the crunchier side, you can throw them in without precooking them. If you like them on the softer side, microwave your green beans for 4-5 minutes before proceeding to the next step.
Before adding your thawed green beans to the pan, make sure you have dried them completely. I actually pat them with a kitchen towel after draining, just to make sure there's no more water.
Toss green beans with the sauce and cook for a minute or two. Pour mixture into your prepared baking dish.
Sprinkle onion evenly over the top. Bake at 350F degrees, uncovered, for 30 minutes.
Let it cool for about 10 minutes before serving so the sauce has a chance to thicken up a bit more as it rests.