Preheat Oven to 375˚F. Set a large pot with water over high heat and bring to a boil then add trimmed green beans. Cook/blanch them for 5 minutes for firmer beans or 8 minutes for softer beans. Drain and set aside. 

Meanwhile, Place a large pan over medium/high heat and add 1 Tbsp butter and 1 Tbsp oil, then add sliced mushrooms with a pinch of salt and pepper. Saute until the mushroom liquid has evaporated and mushrooms are golden.

Move mushrooms to one side of the pan and add chopped onion. Saute onions over low heat for a couple of minutes then stir them together with mushrooms and saute until onions are soft (6-8 min). Add minced garlic and cook for 30 seconds longer while stirring. 

Add 2 Tbsp flour and stir until the flour turns into a paste. Continue cooking 1-2 minutes, stirring constantly until it smells slightly nutty, being careful not to burn the mixture.

Add the 1/2 cup chicken stock and bring to a boil over medium heat, stirring constantly and scraping the bottom of the pan with a spatula. Add 1 cup heavy cream, 1 Tbsp Worcestershire sauce, 1 Tbsp soy sauce and cook over low until sauce has a creamy and gravy-like consistency (it thickens more as it bakes). Season with salt to taste.

Add blanched green beans to the mushroom sauce and stir to combine then transfer to a casserole dish. 

In a separate bowl, combine 1/2 cup breadcrumbs, 1/3 cup parmesan cheese, 1/4 tsp garlic powder and 1 tsp dried parsley. Sprinkle over the green bean casserole. Bake uncovered at 375 F for 20 minutes then broil for 2-3 minutes until topping is golden.