Preheat oven to 190C/375F. Cook trimmed green beans in a large pot of boiling water for 5 minutes for firmer beans and 8 minutes for softer beans. Drain and set aside until needed.
While the beans are cooking, mix the breadcrumbs with grated Parmesan cheese, dried parsley and garlic powder. Set aside for later.
In a large pan heat olive oil with butter and saute sliced and seasoned with salt and pepper mushrooms over medium heat until golden and the liquid has evaporated. Move the mushrooms to the side to free some space on a pan and add chopped onions. Saute for 7-10 minutes longer over low heat, then add pressed garlic and cook for a half a minute longer.
Add flour and stir the mixture together until the flour has mixed in with the fat, continue cooking for a minute or so. Add chicken or vegetable stock and scrape down the bottom of the pan with a spatula to release the burned bits into the pan, add the double/heavy cream, Worcestershire sauce and soy sauce, bring to a boil and let it simmer until the sauce is thickened. (The sauce will get thicker in the oven.) Add more salt if necessary.
Add the green beans to the mushroom cream sauce and pour the mixture into a oven proof dish. Top with the Parmesan breadcrumb topping and bake in the preheated oven for 20 minutes, then broil/grill for 2-3 minutes longer until the topping is golden. Make sure to watch carefully this step as oven temperatures vary and the breadcrumbs it may burn quickly.