Easy Vegan Green Bean Casserole

Make onion topping: Preheat oven to broil on medium and line a medium baking sheet with  aluminum foil. In a medium bowl, toss onion with olive oil and season with salt and pepper. Add flour and Panko and toss to coat onions. Broil, tossing every 2 to 3 minutes, until onions are crisp and golden, about 6 to 8 minutes in total. Turn oven down to 375°. Prepare an ice bath. In a large pot of boiling water, add green beans and cook until bright green, about 6 minutes. With a slotted spoon or tongs, quickly transfer green beans to ice bath to cool, then drain and transfer to a large bowl.  In a large ovenproof skillet over medium heat, heat 2 tablespoons olive oil. Add shallots and cook, stirring occasionally until tender, about 5 minutes. Add mushrooms and season with salt and pepper. Cook, stirring often, until mushrooms are golden, about 5 minutes more. Stir in garlic then transfer mixture to the bowl with the green beans.  Heat remaining 4 tablespoons of olive oil in the same skillet over medium heat. Whisk in flour and cook until golden, about 2 minutes. Gradually whisk in almond milk and bring to a simmer. Cook until thickened, about 4 minutes. Remove from heat then add green bean mixture and toss until even combined. Transfer mixture to a medium casserole dish.  Bake until warmed through and bubbling around the edges, about 20 minutes. Top with "fried" onions and bake 5 minutes more.