Prepare the green beans: Bring 6 quarts of water to a boil in a large pot. Add 2 tbsp. salt to the water and stir in the green beans. Cook for 2-3 minutes. Fill a large bowl halfway with ice cubes and add some cold water. Place a colander squarely inside the ice bath. When the beans are cooked, remove them from the hot water with a strainer and plunge them to the colander in the ice bath. "Swirl" the beans around and submerge them in the ice water so they cool quickly. Drain. Make the mushroom base for the casserole: Melt the butter in a 12-inch skillet over medium heat. Add the garlic, cayenne, white button and shiitake mushrooms. Season with salt to taste and cook until the mushrooms give off most of their liquid, 8-10 minutes. Stir the flour into the mushrooms. Add the chicken stock. Stir in the cream and sour cream and simmer gently. Continue cooking over low heat until the mixture thickens, 5-8 minutes. Add in the mustard. Taste for seasoning. Make the pecan crust: In a medium bowl, stir together 1/2 cup of the pecans, the breadcrumbs, Parmesan cheese, butter and salt. Set aside. Assemble the casserole: Stir the green beans and the remaining 1/4 cup of pecans into the mushrooms. Simmer on the stove, over low heat, for 8-10 minutes, until the green beans become tender when pierced with the tip of a knife. Top with an even layer of the pecan crust. Transfer the skillet to the oven for 10-12 minutes to brown the crust.
You can substitute yellow wax beans for green beans or even mix in some string beans.