Preheat oven to 350° and lightly oil a casserole dish.
Mix together the cashews, lemon juice, nutritional yeast, non-dairy milk, and 1/2 teaspoon of salt in a high-speed blender or food processor until smooth and creamy. Set aside.
Heat the oil in a large pan over medium high heat. Add the onion and cook for about 10 minutes, until it begins to caramelize.
Add the garlic, mushrooms, and green beans to the pan along with the other 1/2 teaspoon of salt. Cover and let cook for about 10 to 15 minutes, stirring occasionally, until the vegetables are tender.
Remove the vegetables from the heat and mix in the cashew cream. Pour the mixture into the casserole dish.
Cover with foil and bake for 20 minutes. Remove the foil and spread the panko or crispy onions in an even layer over the casserole. Bake for 10 more minutes or until the top is golden and the casserole is bubbly.