Preheat the oven to 350 degrees F (175 degrees C).
Melt butter in a large saucepan over medium heat. Add onion and cook until translucent, 5 to 7 minutes. Add carrots and celery; saute until carrots are just beginning to soften, about 3 minutes. Season with salt and pepper.
Lower heat and add turkey; cook and stir for 5 minutes. Add mashed potatoes, cream, and gravy; cook and stir until heated through, about 5 minutes. Pour into a 9-inch pie dish and top with biscuits.
Bake in the preheated oven until golden brown, about 10 minutes. Remove from the oven and let stand for 5 to 10 minutes before serving.