Preheat oven to 350°F. In a medium sized pot, bring salted water to boil. Add green beans, cook 5-6 minutes until tender-crisp. Drain water and set green beans aside.
In a large bowl, whisk together mushroom soup, half & half, garlic powder, salt, and pepper. Stir in cooked green beans, onion, 2 cups cheese and ¾ cup fried onions.
Pour mixture into a 10x12 baking dish. Top with remaining ½ cup cheese. Bake until casserole is bubbly and crusty, about 35 minutes. Remove from oven and top with remaining ¼ cup fried onions. Bake for 10 more minutes. Remove from oven and let sit 5 minutes before serving.
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Cover with foil when baking to prevent fried onions from over browning.
Green Bean Casserole can be made 3 days ahead of time. Cover and store in refrigerator. Reheat in oven at 300°F for 10 minutes before serving.