Cut Velvetta into small squares, set aside; Grease 13x9 pan with cooking spray; preheat oven to 375 degrees; melt 1/3 cup butter for topping, crush ritz crackers, add to melted butter set aside
Melt 5 tbsp butter over low heat, when melted add flour, seasoning salt, pepper, nutmeg. Stir constantly until smooth and bubbly. Gradually add 5 cups milk, when you reach desired consistency, add Velvetta, remove from heat and stir until melted. Add the cooked casserole elbows, transfer to pan, sprinkle topping. Cook for 20 minutes or until topping is golden brown.