Bring a large pot of salted water to a boil over high heat and prepare an ice bath in a large bowl. Blanch the green beans in the boiling water for 2-3 minutes, then transfer to the ice bath to halt cooking. Allow the green beans to cool completely in the ice bath, then drain well and set aside.

In a large saute pan, heat 1 tbsp butter and 1 tbsp olive oil over medium heat. Add half the sliced mushrooms to the pan with a pinch of salt, toss to coat, then arrange in a single layer and allow to brown undisturbed for the first 5 minutes or so of cooking (this will get you the best, more caramelized brown!) Stir and cook a few minutes more, then transfer to a dish and set aside. Add another ½ tbsp of butter and ½ tbsp oil to the pan and repeat the process with the remaining mushrooms. Each batch will take about 7-8 minutes to cook.

When the second batch of mushrooms has finished browning, transfer them to the dish with the previously browned mushrooms. Add another ½ tbsp butter and 1 tbsp oil to the pan along with the diced onions, and saute until translucent, about 8 minutes. Sprinkle the flour over top of the onions and cook 1 minute while stirring.

Continue to stir, and slowly add the cream and milk to the pan. Stir in the worcestershire sauce and black pepper to taste. Allow the sauce to come up to a simmer, then begin to sprinkle in the parmesan while stirring, waiting for each addition of cheese to melt completely before adding more.

When all of the cheese has been incorporated, add the mushrooms and the green beans to the sauce and toss to coat. Transfer to a 13” baking dish and cover with a lid or aluminum foil. Bake at 350F for 45 minutes, then uncover, top with the crispy onions, and bake 10 minutes more.