In a large pan or skillet, fry the mushrooms in a tablespoon of olive oil until golden brown.
Add the garlic and cook until fragrant.
Sprinkle in the flour and stir to coat the mushrooms then pour in the cream and milk.
Whisk until the sauce is smooth then season with nutmeg, salt and pepper. Allow to simmer for 5 minutes until smooth and thickened.
Bring a large pot of salted water to the boil over medium high heat. Trim the green beans by cutting off the tips and halving them.
Add the beans to the boiling water and allow to simmer for 2 minutes then drain and rinse with cold water.
Melt the butter in a pan then add the bread crumbs and onions. Toast for a minute or two then remove from the heat and add the Parmesan.
Place the green beans in the bottom of a casserole dish or oven proof baking dish. Pour over the mushroom sauce and top with the panko mixture. Place in the oven.
Bake the casserole at 180ºC/350ºF for 20-30 minutes until crisp, golden and bubbling. Remove from the oven and allow to cool for 5 minutes then serve.