Preheat oven to 350°. In a large skillet, saute mushrooms and onion in oil over medium heat until tender. Add garlic; cook and stir 1 minute. Stir in the sour cream, browning sauce (if desired), salt and pepper until blended. Add green beans and cheese; stir to coat. Transfer to a greased 13x9-in. baking dish. , In a small bowl, melt buttery spread. Stir in crushed crackers and fried onions; sprinkle over casserole. Bake until casserole is bubbly and top is golden brown, 25-30 minutes.