How to Make It
Preheat oven to 350°F. Stir together 1 cup panko (Japanese breadcrumbs), 1 cup untoasted sliced almonds, 1/2 tsp. kosher salt, and 2 Tbsp. melted salted butter. Set aside.
Squeeze green beans lightly in hands to remove excess water. Heat 2 tablespoons of the butter in a Dutch oven over medium-high. Add chopped fresh shallot, and cook, stirring often, until shallots are caramelized but not burned, 8 to 10 minutes. Transfer shallots 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. Whisk in the lemon lemon juice. Cook, whisking constantly, until thickened and bubbly, about 4 minutes. Remove from heat, and whisk in Parmesan cheese, 2 remaining salt, and pepper until melted and smooth. Stir in green beans, shallots, water chestnuts, and toasted almonds; spoon into a lightly greased 13- x 9-inch (3-quart) baking dish.
Sprinkle panko mixture over casserole, and bake in preheated oven until bubbly and topping is golden brown and crisp, about 30 minutes.