Preheat your oven to 375°F (190°C).
Steam or blanch the green beans until they are tender but still crisp. Drain and set aside.
Heat some olive oil in a large skillet over medium heat. Add the sliced onions and sauté until they become caramelized and golden brown. This will take about 10-15 minutes. Add the minced garlic and cook for an additional minute.
In a separate bowl, whisk together the vegetable broth, non-dairy milk, and gluten-free flour until well combined. Pour this mixture into the skillet with the onions and garlic. Stir well and cook for a few minutes until the sauce thickens.
Add the nutritional yeast, dried thyme, dried rosemary, salt, and pepper to the skillet. Stir until the ingredients are evenly distributed.
Place the steamed green beans in a baking dish and pour the sauce mixture over them. Toss gently to coat the green beans.
In a small bowl, combine the gluten-free breadcrumbs with a drizzle of olive oil. Mix until the breadcrumbs are coated.
Sprinkle the breadcrumb mixture evenly over the green beans.
Bake in the preheated oven for about 20-25 minutes or until the casserole is bubbly and the breadcrumbs turn golden brown.
Remove from the oven and let it cool slightly before serving.