In a saucepan over medium heat, melt 2 tablespoons butter, and stir in the flour, salt, paprika, and pepper until smooth. Gradually blend the milk into the saucepan until thickened.
Place the onions and nuts in a medium casserole dish.
Pour the sauce over the onions and nuts, and toss to coat.
In a separate saucepan, melt the remaining butter, and toss with the bread crumbs to coat.
Sprinkle over the casserole.
Bake 20 minutes in the preheated oven, until bubbly and lightly browned.