Preheat oven to 350 degrees F.
Add half of the coconut oil in a large skillet and place over medium-low heat. Add sliced shallots and stir often to prevent burning for 30 to 40 minutes, or until caramelized. Once the onions are golden, remove from heat, transfer to a bowl and set aside.
While the onions are cooking, steam the cauliflower florets until softened to the point they can be easily pierced with a fork. Transfer to a high-speed blender OR food processor and set aside.
Steam the green beans for 6 to 8 minutes or until bright green. Blanch in cold water, drain and transfer to a casserole dish.
Heat the remaining coconut oil in a skillet over medium-low heat. Saute mushrooms for 5 minutes or until softened and then add the garlic. Saute for another minute and remove from heat. Spread half of the mushroom mixture over the green beans. Add the remaining half to the blender with the steamed cauliflower.
Add water, sea salt and nutritional yeast to the blender. Blend until smooth. Be patient, it takes some time to reach a creamy consistency.
Pour the desired amount of creamy cauliflower sauce into the casserole dish over the green beans and mushrooms. You may not need all of the sauce. Smooth the top with a spoon or spatula and top with caramelized onions.
Bake the casserole for 30 minutes. Remove from oven and let sit 5 minutes before serving.
Garnish with toasted slivered almonds.