Add ½ cup cashews to a bowl with water for soaking. Let soak for at least 30 minutes.
Preheat the oven to 475°F.
Combine the onions, flour, panko and salt in a large mixing bowl and toss to combine. Coat a sheet pan with nonstick cooking spray and evenly spread the onions on the pan. Place the pan on the middle rack of the oven and bake until golden brown, approximately 25 minutes, tossing every 10 minutes. Once done, remove from the oven and set aside until ready to use. Reduce oven temperature to 375°F.
While the onions are cooking, prepare your green bean mixture.
Bring a large pot of water to a boil. Add the beans and blanch for 5 minutes. Drain in a colander and immediately plunge the beans into a large bowl of ice water to stop the cooking. Drain and set aside in large bowl.
Drain cashews and add to a blender with almond milk. Blend until smooth and creamy and set aside.
Melt the butter in a large skillet over medium-high heat. Add the mushrooms, salt and pepper and cook, stirring occasionally for 4 to 5 minutes. Add the garlic and nutmeg and continue to cook for another 1 to 2 minutes. Sprinkle the flour over the mixture and stir to combine. Cook for 1 minute. Add the broth and tamari, simmer for 1 minute. Decrease the heat to medium-low and add the cashew cream. Cook until the mixture thickens, stirring occasionally, approximately 6 to 8 minutes.
Remove from the heat and pour sauce into a large bowl with the green beans and ½ cup of the crispy baked onions. Place green bean mixture into a 8x8 baking dish and top with the remaining crispy baked onions. Place into the oven and bake until bubbly and warm throughout, approximately 15-20 minutes. Remove and serve immediately.