In a saute pan, add 2 tablespoons butter and heat on high. Then add in the shallots. Reduce to medium heat with lid on and cook for 10 minutes, mixing often so that the shallots don't burn. Half way through cooking add the water and mix well. Cook until golden brown and mostly crispy. Remove the crispy shallots from the pan and set to the side.
Place the remaining 2 tablespoons butter in the sauté pan, melt, and add the haricot vert and diced mushrooms in the pan with ½ cup water. Season with salt and pepper. Cook on high for 8 minutes until tender and most of the liquid has evaporated. In a measuring cup (or bowl) mix together the heavy cream and corn starch.
Pour the heavy cream/corn starch mixture into the pan. Turn the heat off and stir. Cover and let sit for 2 additional minutes.
Place the green bean casserole in a serving dish, and top with crispy shallots. Enjoy.