• Preheat the oven to 350 degrees. In a large skillet, melt butter over medium heat.
• Saute onions and mushrooms until just tender, stirring occasionally. Add garlic and cook for another 1-2 minutes, then remove from heat and set aside.
• In a medium saucepan, combine green beans with chicken broth. Bring to a boil.
• Continue cooking for about 8 minutes until beans are crisp-tender and drain.
• Prepare a 2-quart baking dish by spraying it with cooking spray. In a large bowl, combine the soup, mushroom mixture, green beans, Parmesan, salt, and pepper.
• Transfer the mixture to the prepared baking dish and top with crispy onion strings.
• Bake at 350 degrees for 20 minutes, or until the casserole is heated through.

Notes:

• Don’t overcook the green beans: While you are blanching the green beans, keep in mind that they will continue to cook and soften while baking in the oven with the rest of the casserole. In order to avoid them getting mushy in the oven, it’s important to slightly undercook them during this first step. I usually boil my beans for about two minutes since I like them crunchy.
• Pre-toast the breadcrumbs: To make the Panko breadcrumbs extra crispy and golden, sauté them briefly in butter before baking them in the oven.
• Very thinly slice the red onion: Additionally, I recommend slicing the onion as thinly as possible, so that it can crisp up well in the oven.
• Feel free to make the casserole ahead of time: To prepare Ina Garten Green Bean Casserole ahead of time (very helpful for Thanksgiving), just follow the recipe instructions through Step 5. Instead of placing the baking dish in the oven, cover it tightly with foil or plastic wrap and refrigerate for up to 48 hours. When ready to bake, uncover and proceed as directed.