Cook the macaroni according to package directions.
Melt the 1 T butter in a saucepan;
stir in the onions and saute until they are translucent.
Stir in the flour and cook a minute or so, stirring.
Slowly add the milk, stirring constantly.
When all the milk is in and the sauce is smooth and creamy, stir in the salt, pepper, the 1 and 1/2 c cheese, the worcestershire and dry mustard.
Reheat a bit if necessary to melt all the cheese.
Stir in the cooked macaroni.
Now comes the au gratin part:
Turn the macaroni and cheese into a greased 2 quart casserole.
Combine the bread crumbs and the 1/2 c cheese and sprinkle over the surface of the M&C.
Melt the 3 T butter in a small pan and drizzle over all.
Bake at 350 for 10 minutes or so (unless the mixture had gotten cold before it went into the oven, in which case make it 20 or 30 minutes), And run it under the broiler quickly at the end to brown the top.