Begin by melting half a tablespoon of coconut oil or butter in a large saucepan over low heat. Add the sliced red onions and cook slowly, stirring occasionally, for 30 to 45 minutes until caramelized. If the onions begin to dry out, add a small splash of water to prevent burning.

While the onions are caramelizing, prepare the parsnips by steaming them in a steamer basket for approximately 10 minutes or until they are soft enough to be pierced with a fork. Allow the parsnips to cool briefly after steaming.

Trim the ends off the fresh green beans, then cut them into 1½ to 2-inch pieces. Place the green beans in the steamer and steam for about 8 minutes until they are tender but still slightly crisp. Transfer the steamed green beans into an 8x8-inch baking dish and set aside.

Once the onions have finished caramelizing, remove them from the pan and add the remaining coconut oil or butter to the same pan over medium-high heat. Add the mushrooms and sauté them for one minute before adding the minced garlic. Continue cooking until the mushrooms are softened, approximately 5 minutes. Pour half of the mushroom-garlic mixture over the green beans in the baking dish.

Preheat the oven to 350°F (175°C) in preparation for baking the casserole.

In a blender, combine the steamed parsnips, remaining mushroom-garlic mixture, water, salt, and nutritional yeast. Blend until smooth and creamy. Add pepper to taste and stir in crushed red pepper if desired for additional spice.

Pour the parsnip puree over the green beans and mushrooms, stirring to coat the vegetables evenly. You may not need to use the entire puree mixture, depending on desired consistency. Place the caramelized onions on top as a final layer.

Bake the assembled casserole in the preheated oven for approximately 30 minutes until the flavors meld and the casserole is heated through. Allow it to cool slightly before serving.