Melt 3 T. butter in a large skillet. Sauté the onions, stirring, for 5 minutes. Remove from heat.
Blanch the celery in lightly salted, boiling water for 3 minutes. Drain and refresh in cold water. Drain thoroughly and stir into the onions.
Melt 3 T. butter in a heavy saucepan and stir in the flour. Cook over medium, stirring, for 2 minutes without browning.
Whisk in the milk, whisking until mixture thickens. Season with salt and pepper and fold into the vegetable mixture. Spoon into a greased 2-quart baking dish.
Melt the remaining 2 T. butter in a skillet. Stir in the paprika, then the bread crumbs. Stir to coat the crumbs. Remove from heat and stir in the parsley.
Sprinkle the breadcrumb mixture over the top of the vegetables.
Bake in a 350 °F. oven for about 25 minutes or until bubbly.
Serves 8 to 10