Preheat your oven to 350°F (175°C). Lightly grease a 9x13 inch baking dish.

In a large skillet, melt the butter over medium heat. Add the chopped onion and celery and cook until softened, about 5-7 minutes. Add the sliced mushrooms and cook until they have released their liquid and started to brown.

Sprinkle the flour over the cooked vegetables and stir constantly for one minute.

Gradually whisk in the chicken broth until the mixture is smooth. Stir in the heavy cream and sherry. Bring to a simmer and cook for 2-3 minutes, stirring occasionally, until the sauce has thickened.

Remove the skillet from the heat. Season with salt and pepper. Gently stir in the cooked chicken until everything is well combined.

Pour the mixture into the prepared baking dish and spread evenly.

Sprinkle the slivered almonds over the top of the casserole and dust with paprika.

Bake for 25-30 minutes, or until the casserole is hot and bubbly and the almonds are toasted and golden.