To make sauce, melt butter in small saucepan over low heat. Stir in flour, cook 1-2 minutes, stirring constantly. Using a wire whisk, stir in milk a little bit at a time. Bring to a boil. Cook, whisking constantly, 1-2 minutes until thickened. Stir in ranch and white pepper; set aside.
2.
Preheat oven to 350 degrees. Grease a skillet and heat over medium high heat. Add onion and garlic, cook and stir until tender, about 3 minutes. Remove half and set aside.
3.
Add mushrooms to skillet with onions and cook about 5 minutes. Combine mushroom mixture, green beans, and sauce in a casserole dish. Combine fried onions with reserved onion mixture and sprinkle over casserole dish. Bake uncovered 20-30 minutes.
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