Wash and trim stems from the green beans and place in a pot of boiling water. Cook for about 5 minutes until green beans begin to tender and then drain into a colander. Rinse with cold water to prevent overcooking. Transfer green beans to a greased 9"x13" baking dish.
Preheat oven to 350 degrees.
Add 2 tablespoons of butter and sliced onions to a large skillet and bring to medium heat. Cook for about 5 minutes stirring occasionally until onions begin to tender. Add sliced mushrooms and minced garlic and cook for a few minutes until garlic is fragrant and mushrooms are tender. Pour mushroom and onion mixture over the green beans.
In the same skillet you cooked the onion mixture, add 2 tablespoons of butter, flour, and salt over medium heat. Stirring frequently, slowly add milk. Stir in pepper, worcestershire sauce, and mustard and then remove from heat. Pour sauce into the baking dish with green beans and toss to combine.
Evenly sprinkle cheddar cheese and crispy onions on top of casserole. Place in the oven and bake for about 30-35 minutes until top is nice and golden and sauce is bubbly.