Boil a pot of water and once boiling, add a heaping pinch of kosher salt. Add green beans and cook for 1 minute. Add to ice bath to stop cooking.
Prepare your topping by combing all ingredients in a bowl. Set aside and preheat oven to 375 degrees.
Heat a medium skillet over medium heat. Add mushrooms and toss to coat. Allow mushrooms to cook in an even layer to release their liquid. About 15-20 mins.
Next, add onions and cook until they are softened and fragrant. About 3 mins. Add garlic, worcestershire sauce, sherry vinegar and cook for another 2 mins.
Add butter and allow to melt. Once melted, add in flour and cook for 3-5 mins, just until raw flour taste is cooked out.
Slowly, pour in 1/3 of the stock, whisking continuously to avoid lumps. Add the rest of the stock in a slow and steady stream while whisking. Follow with half & half and stir until the sauce can coat the back of a spoon.
Add cheese and stir to melt. Remove from heat and season with black pepper, nutmeg, cayenne and taste for any additional seasoning. Depending on the stock used, you may need to add salt as well.
Drain green beans and pat as dry as possible. Stir to coat beans and arrange in a flat layer. Add prepared topping and put in oven.
Bake at 375 for 15-20 mins, or until top is browned and casserole is bubbling. Allow to cool for at least 5-10 mins. Enjoy!