1poundsliced shiitake, cremini, or button mushrooms (or a combination)
1large shallot, thinly sliced
2cupshalf and half or whole milk
1cupchicken or vegetable broth (preferably low-sodium)
2teaspoonsWorcestershire or soy sauce
Kosher salt and freshly ground pepper to taste
¾cupprepared fried onions or shallots
Preheat the oven to 350°F. Grease a 9x13-inch or 3-quart shallow baking dish.
In a large deep pan or skillet melt the butter over medium high heat. Add the mushrooms and sauté them until they have released their liquid, the liquid evaporates, and the mushrooms start to turn golden brown at the edges, about 8 minutes. Add the shallots and sauté for another 3 minutes until the shallots become tender and golden and the mushrooms brown a bit more. Stir in the flour and cook, stirring often for another 2 minutes until the flour is well incorporated.
Slowly add the milk and broth, stirring, and bring the liquid to a simmer. Add the soy or Worcestershire sauce. Reduce the heat to medium low and continue cooking the mixture at a gentle simmer for about 5 minutes until the sauce has thickened. Stir in the drained green beans, season with salt and pepper, and transfer the mixture to the prepared baking dish.
Bake for about 20 minutes until the top is golden brown and the casserole is bubbling. Evenly distribute the fried onions on top of the casserole, and bake for another 5 minutes to heat and toast the onions. Serve the casserole hot or warm.