Fill a large pot with water. Cover and place over high heat until the water boils. Add the green beans and reduce the heat to medium. Cook the beans for 3-5 minutes or until crisp tender. Place in a colander and rinse well with cold water to cool the beans down (this will help the beans retain their green color). Drain well and set aside.

Heat 1 tablespoon of butter in a large saute pan over medium heat. Add the mushrooms and cook until softened and the liquid has evaporated, about 5-6 minutes. Add the garlic and cook 30 seconds more.

Add the remaining 2 tablespoons of butter and melt. Sprinkle 2 tablespoons of flour evenly over the butter-mushroom mixture. Stir and cook for 1 minute or until the flour smells like shortbread.

Add the milk slowly while whisking constantly (be sure to scrape the bottom of the pan as well), then whisk in the dijon mustard, Worcestershire sauce and salt and pepper. Continue to whisk constantly and bring the milk to a boil. Continue cooking until the sauce has thickened and it coats the back of a wooden spoon, about 5 minutes more.

Remove from heat and stir in the cheese until melted, then stir in the green beans and toss to coat. From here, you can place the mixture in a bowl and store in the fridge until you're ready to top with the fried shallots and bake in the oven.

Preheat the oven to 350 degrees. Grease a 2 – 2 1/2 quart baking dish with cooking spray. Pour the green bean mixture into the baking dish and set aside.

Place the shallot rings in a bowl and toss with the remaining 2 tablespoons of flour and about 1/2 teaspoon salt.

Place the oil in a medium sauce pan and heat over medium high heat. When the oil is hot (not smoking), add about 1/4 of the shallots in a single layer and cook 2-3 minutes until lightly golden brown. Place on a paper towel lined plate. Repeat until all of the shallots have been cooked. Sprinkle with a bit more salt if necessary.

Top the casserole with the shallots and bake at 350 degrees for 30-40 minutes or until bubbly and heated through (this will take longer if the green bean mixture is cold). If the onions begin to brown too quickly, cover with foil.

Remove from heat and cool 10 minutes. Serve and enjoy!