Melt butter over medium heat in a medium-large sized pot. Add onion and celery, and saute 1 minute. Add flour and cook, stirring, 1 minute more. Add milk or cream in a gradual stream, stirring constantly to incorporate. Stir in chicken broth and soy sauce. Cook until simmering and thickened, about 6 minutes.
In a large bowl, combine turkey, bean sprouts and 3/4 of the chow mein noodles. Pour the creamy sauce over top, and toss to thoroughly combine. Taste and season with salt and pepper.
Spread mixture in a baking dish (of 2 1/2 - 3 quart/litre capacity; 9x13" works well) and sprinkle with reserved chow mein noodles. Cover with aluminum foil and bake 30 minutes, or until hot and bubbly.