; Boil the green beans for 3-5 minutes (photo 1).Drain and rinse with cold water to stop the cooking. Along with these 5 vegan casseroles to bring to a potluck, this vegan green bean casserole is something you can't miss during the holiday season.In this blog, we provide you a perfect vegan green bean casserole recipe with creamy mushroom sauce, crispy fried onions, and a crunchy breadcrumb topping. ; Add the onion and garlic and cook over medium-high heat for about 3-5 minutes (photo 3), stirring occasionally. Caramelize the Onions: Bring a large, nonstick pot to medium-high heat and add the onion along with 1-2 tablespoons of water.Stir the onions occasionally; you will notice the edges of the onions starting to brown, as well as brown marks on the bottom of the pan. Prep: Preheat the oven to 375F and set a 8×8″ (or similar sized) casserole dish to the side. For the green beans, this version uses the canned variety which saves time spent washing and cutting and helps cut down on the cooking time of the casserole. Drain and set aside. ; Heat the vegan butter in a large skillet (photo 2). Instructions. This version uses gluten-free crispy onions (aka French fried onions). Preheat the oven to 400ºF or 200ºC. This version features nutritional yeast and tamari, which add an umami flavor to the dish.