Bring a large pot of salted water to a boil. Add the beans and blanch until crisp but tender, 2-3 minutes. Immediately drain the beans and run cold water over them or plunge them into an ice bath to cool and stop the cooking process. Drain and dry.

In a large skillet, melt the butter and add the mushrooms. Season with salt and pepper, to taste, and add mustard. Cook the mushrooms, 5-8 minutes until they give off most of their liquid.

Using a whisk, add the garlic and flour to the mushrooms. Stir to incorporate then add the chicken broth. Bring to a simmer. Stir in the cream and sour cream, and bring to a simmer, tasting to adjust seasoning. Cook mixture until it’s thickened and coats the back of a spoon, about 10 minutes.

To assemble, stir the green beans into the mushroom mixture. Simmer on the stove, until the green beans become tender.