1Heat oven to 350°F. Spray 11x7-inch or 8-inch square (2-quart) glass baking dish with cooking spray.
2In 12-inch skillet, melt butter over medium heat. Add flour; stir with whisk until blended. Gradually stir in half-and-half and Worcestershire sauce; cook and stir until smooth and bubbly. Add mushrooms and garlic; cook and stir until sauce begins to thicken. Remove from heat.
3Add green beans; toss until beans are coated with sauce. Pour mixture into baking dish. Top with onions.
4Bake uncovered about 30 minutes or until beans are tender and onions are golden brown.