Preheat oven to 350 degrees fahreneheit.
Bring a large pot of water to a boil, and cook green beans for 3 minutes. Drain and run under cold water until cool. Set aside.
Place cashews in a heat safe bowl or glass measuring cup, and cover with very hot water. You can ladle out boiling water from the green beans for this, if you'd like, or just heat water separately in a kettle on the stove. Soaking the cashews will make them soft, and easier to blend.
Remove and discard the stems from your mushrooms, and dice the mushrooms. Place a large skillet on the stove over medium heat and add mushrooms and olive oil. Sautee for about 5 minutes, until starting to brown.
Season mushrooms with salt, pepper, garlic powder, and thyme.
Drain the cashews, and place in the bowl of a high powered blender or food processor. Add broth, and blend until completely smooth.
Pour cashew sauce into the pan with the mushrooms, and cook for another 5 minutes, until thick and creamy. Taste and add more salt, if needed (this should be slightly salty, as it will season the green beans as well).
Place green beans in a large baking dish, top with sauce, and stir to combine. Top with crispy onions, and bake for 25 minutes, or until deeply golden.