Sauté the onion in the tablespoon of butter. Add the flour and cook, stirring, for 2 minutes without browning. Whisk in the milk and cook until thickened. Stir in the seasonings.
Add the chicken and spaghetti.
Sauté the mushrooms in the 2 tablespoons butter and add to the mixture.
Fold in the cheese.
Put into a buttered dish and top with the crumbs.
Bake in a 425 °F. oven for 15 minutes or until hot through.