Preheat oven to 375 degrees Fahrenheit
Prepare a large baking dish with cooking spray, approx. size 9x13 and set aside
Bring water and a pinch of salt to a boil in a large pot. Add beans, and bring back to a boil. Reduce heat to medium and cook 5-6 minutes until beans are tender but not too soft.
While beans are cooking, place breadcrumbs and salt and pepper on a large plate. Dredge onions in the scrambled egg (drop the onions into the egg mixture and then lift out, letting excess egg drain off, drop onto the plate of breadcrumbs and coat).
In a large saute pan, heat 2 tsps of olive oil over medium high heat. Add coated onions and lightly fry until onions are golden brown and crispy.
Transfer to a plate with paper towel to drain excess oil. Set aside.
Once green beans are done cooking, drain and pour into a bowl of ice water so they stop cooking.
In the same saute pan used for the onions, add another 1 tsp of olive oil and add mushrooms. Cook until golden brown.
Reduce heat to low and add flour. Stir to coat the mushrooms in flour. Slowly pour cashew (or almond) milk and whisk while pouring, until no lumps are left. Add water, Greek yogurt, Parmesan cheese, salt and pepper and whisk again.
Bring to a boil and keep stirring until sauce begins to thicken. This will take a few minutes.
Once sauce is thick, drain the green beans and add to sauce, coating until every green bean is covered. Transfer to your prepared baking dish.
Top with fried onions and bake for 25-30 minutes until heated all the way through.
Serve immediately while warm!