Wash the fresh green beans, remove the ends, and cut them in half. Partially cook the green beans by blanching them for 5 minutes and then shocking them in ice water for 5 minutes. You can also steam them in the microwave (Place the green beans in the microwave with 2 tablespoons of water, cover with a microwave-safe plate and cook for 5 minutes).
Preheat the oven to 400°F/200°C.
To make the mushroom sauce, in a saucepan or pan melt the butter over medium heat.
Add the mushrooms, season with salt and pepper and cook until they start to soften.
Add garlic, and cook for 30 seconds then add the flour and cook for 30 more seconds. Add the flour, and cook for 30 seconds to 1 minute.
Gradually add the stock, about ½ a cup at a time and keep stirring until all the stock is in. Stir in the heavy cream, and simmer for 1 more minute. Stir in the nutmeg, taste the sauce, and adjust the salt and pepper to your preference. Remove from heat.
Assemble. In a casserole dish, mix the green beans with the sauce and bake for 15 minutes or until bubbly.
Remove from the oven, top with the crispy onions, and bake for 5 more minutes (the onions will change their color a little to golden brown, and become even more crispy).
Gluten Free. If you're making a gluten free green bean casserole, use gluten free flour and omit French's fried onions as they're not gluten free.
Making Ahead. You can follow the recipe all the way up until the step where you add the fried onions. Stop, wrap up the casserole dish, and put it in the fridge for up to 3 days until you’re ready to bake it. About 20 minutes before you want to eat, top the green bean casserole with french fried onions and bake until it’s ready.
Double Batch. I suggest that if you want to double the recipe, that you bake it in two separate pans so that it cooks evenly. A deeper casserole dish may seem like a good idea, but you’ll miss out on all of that crispy onion surface area that way!