How to Make It
Preheat oven to 350°F. Squeeze green beans lightly in hands to remove excess water.
Cook bacon slices in Dutch oven over medium until crisp, about 6 to 8 minutes. Drain bacon on paper towels, reserving drippings in Dutch oven. Crumble bacon and set aside.
Cook Brussels sprouts and green beans in reserved drippings in Dutch oven just until tender and lightly browned, about 8 to 10 minutes. Remove vegetables from Dutch oven.
Add 2 tablespoons of the butter to the drippings in the Dutch oven over medium-high. Add mushrooms and chopped fresh shallot, and cook, stirring often, until mushrooms are deeply browned and shallots are caramelized but not burned, 8 to 10 minutes. Transfer mushroom mixture to a plate, and without wiping out Dutch oven, add remaining 6 tablespoons butter to Dutch oven. Melt butter over medium-high; whisk in flour, and cook, whisking constantly, 1 minute. Gradually add milk, whisking constantly until smooth. Cook, whisking constantly, until thickened and bubbly, about 4 minutes. Remove from heat, and whisk in Parmesan cheese, salt, and pepper until melted and smooth. Stir in reserved Brussels sprouts and green beans, mushroom mixture, and water chestnuts; spoon into a lightly greased 13- x 9-inch (3-quart) baking dish.
Bake in preheated oven until bubbly around edges, about 15 minutes. Remove from oven, and sprinkle crumbled bacon with fried onions or shallots over casserole. Return to oven, and bake until bubbly and topping is golden brown and crisped, about 15 minutes.