Preheat oven to 350°. In a large skillet, saute mushrooms and onions over medium heat in oil until tender. Add garlic, cook and stir 1 minute. Stir in the sour cream,salt and pepper until blended. Add green beans and cheese; stir to coat. Transfer to a greased 13×9-in. baking dish.
In a small bowl, melt remaining 1/4 cup buttery spread. Stir in crushed crackers and fried onions; sprinkle over casserole. Bake until bubbly and top is golden brown, 25-30 minutes.