Preheat the oven to 375 degrees and butter 2 8-inch baking dishes or 1 9x13 baking dish.
Cover the dried mushrooms with 1 cup of boiling water and let stand while you assemble the rest of the ingredients. Squeeze out the excess water from the mushrooms and finely chop. Reserve the soaking liquid.
Bring a large pot of salted water to a boil and blanch the green beans for 4 minutes. Drain and immerse the beans immediately into cold water to stop the cooking. Drain and set aside.
In the same large pot, melt the butter over medium high heat and sauté onion and garlic for 3 minutes or until the onion becomes translucent. Stir in the chopped mushrooms and salt and pepper and cook for another 3 minutes. Stir in the flour and cook for 2 minutes. Slowly whisk in ½ cup of the reserved mushroom liquid, then the chicken broth. Sauce should be quite thick at this point. Reduce heat to a simmer and whisk in the heavy cream. Cook and stir for 3-5 minutes until the sauce has reduced and become very thick, like pancake batter. Remove from heat and taste for seasoning, adding more salt and pepper if necessary. Stir in the beans and then divide the mixture evenly between the baking dishes (if you are using 2). Top each with the breadcrumbs, then the onions. Cover loosely with foil and bake for 15 minutes. Remove the foil and bake for another 5 minutes. Serve and enjoy!
Note: Dried mushrooms can be found in the Asian aisle usually, but if you can’t find them use ½ cup finely chopped button mushrooms instead and increase the chicken broth to 1 cup.