Melt butter in a small saucepan. Stir in flour, nutmeg, and 1/8 teaspoon each salt and pepper until smooth.
Gradually add milk. Bring to a boil, and cook until thickened, about 2 minutes.
Remove from heat and allow to cool completely.Meanwhile, cook sausage, onion, and remaining salt and pepper in a large skillet. Drain; and transfer mixture to a greased 13x9 inch baking dish.
In a large bowl, whisk together eggs and half-and-half.
Stir in spinach, feta, and the cooled white sauce. Pour over sausage mixture.
Cover dish and refrigerate overnight.
Remove from refrigerator 30 minutes before baking.
For topping, toss breadcrumbs with the butter and spinkle over the top of the casserole. Sprinkle with parmesan cheese.
Bake, uncovered, at 350 for 45 to 50 minutes, or until a knife inserted in the center comes out clean.
Let stand approximately 10 minutes before slicing.