Melt two tablespoons of butter in a large pot (you could use the one you used for the spaghetti). Add celery and onion, saute until soft. Add all the remaining ingredients. However you may want to chop the chicken into bite sized pieces. Stir well and continue to cook on medium heat until it begins to bubble. Stir often while cooking. Remove from heat and mix in cooked spaghetti. Pour mixture into your prepared pan(s). Top with panko. Place in preheated oven and bake until top is browned and mixture is bubbly, about 50 minutes.