Heat a skillet over medium heat. Add butter. Once melted, add onions and mushrooms. Sprinkle with salt and pepper. Cook until soft, about 5 minutes. Next, add garlic and cook for another minute.
Sprinkle the flour in and stir to combine. Cook for a minute until its soaked up the moisture, then slowly add in the broth and then the milk or half and half. Mix in the Dijon mustard and Worcestershire sauce. Bring to a simmer.
Once simmering reduce the heat to low and stir in the cheese and season with salt and pepper. Simmer for 5-10 minutes until thickened. Taste and add more salt if necessary.
Stir in the green beans and ½ cup of the fried onions. Once combined, pour into a greased 2 - 3 quart casserole dish.
Bake uncovered for 15 minutes, or until bubbly. Remove from oven, add the remaining fried onions and bake for another 10 minutes. (If using canned green beans, skip the first 15 minute bake and just bake for 10 with with fried onions).