Crab Macaroni Casserole

How would a seamaiden make a casserole? She would make crab a main ingredient, of course!

Ingredients
2 cups uncooked elbow macaroni
3 tbsp. onion, diced
2 tbsp. butter
3 tbsp all-purpose flour
1 1/2 cups 1% milk
14 oz. lump crabmeat
1 tbsp. Old Bay seasoning
1 cup reduced-fat sour cream
1/2 cup shredded Swiss cheese
1/2 tsp. salt
1/2 tsp. ground mustard
1 cup shredded cheddar cheese

Directions
Cook macaroni according to package directions.
Saute onion and butter in a large skillet until tender.
Add flour and milk and combine until smooth.
Bring to a boil and cook and stir 1-2 minutes until thickened. Remove from heat.
Add crabmeat, Old Bay seasoning, sour cream, Swiss cheese, salt, mustard, drained macaroni and 1/4 cup of cheddar cheese to skillet.
Transfer to a 12 x 9 baking dish coated with cooking spray.
Sprinkle with remaining cheddar cheese.

Bake at 350 degrees for 20-25 minutes or until heated through.