- Preheat the oven to 400°F . Lightly spray a 9 × 13-inch baking dish with olive oil.
- Place the green beans in a steamer basket over a pot of boiling water and cover. Steam for 5 minutes, then transfer to a large bowl. Pour the soup into the bowl and stir to combine. Set aside.
- While the green beans are steaming, melt half of the butter in a large frying pan over medium heat. Add the onion and cook, stirring occasionally, until soft and golden, 5 to 7 minutes. Transfer the onions to a medium bowl.
- Melt the remaining butter in the frying pan and add the bread crumbs. Cook, stirring frequently, until the crumbs are crispy. Stir in the garlic powder and salt and remove from the heat. Add to the onions along with the nutritional yeast (if using).
- Stir to combine. Pour the green bean mixture into the prepared baking dish. Spread the onion mixture over the top. Bake for 25 minutes, or until the topping is crispy and the casserole is bubbly.
- Serve immediately. Leftovers will keep in an airtight container in the fridge for 3 to 4 days.