Fish & Mushroom Casserole with Crispy Phyllo

Preheat oven to 425 degrees F. Coat a 9-by-13-inch (or similar size 3-quart) baking dish with cooking spray.

Heat oil in a Dutch oven over medium heat. Add onion and celery and cook until softened, 6 to 8 minutes. Add mushrooms and cook until they start to release their liquid, 6 to 8 minutes. Increase heat to medium-high and cook, stirring occasionally, until the mushroom liquid has evaporated, 12 to 14 minutes more.

Whisk sherry and tomato paste together and add to the pot. Cook, stirring occasionally, until reduced by about half, 5 to 7 minutes. Whisk flour and milk together and add to the pot. Reduce heat to medium and cook until thickened, 1 to 2 minutes. Stir in cod, dill, salt and pepper. Cook, stirring often, until the cod is just cooked through, 3 to 5 minutes. Transfer to the prepared baking dish.

Place one sheet of phyllo on a clean cutting board. (If you have smaller sheets, place two side by side.) Coat the phyllo with cooking spray. Cut into quarters. (Cut smaller sheets in half to get 4 pieces.) Using your fingertips, crumple each piece of phyllo so it resembles a little nest and place on the casserole. Repeat with the remaining phyllo, fitting the "nests" snugly together to form a tight layer. Evenly coat with cooking spray.

Bake the casserole until the top is golden brown and the filling is bubbling, 18 to 22 minutes. Let cool for 5 minutes before serving.