Swiss Asparagus Casserole

6 tablespoons butter, divided
2 cups fresh asparagus, cut diagonally in 1 1/2 inch slices
3/4 cup onion sliced - medium onion
1 tablespoon flour
salt and pepper to taste
1 cup sour cream
1 cup Swiss cheese, shredded
1/4 cup Panko crumbs - cracker crumbs work well too

Preheat oven to 450 degrees. In a large saucepan over medium-high heat elt 2 tablespoons butter in large skillet. Saute asparagus for 2-3 minutes until barely tender. Remove from pan and set aside. In same skillet, saute onion in 2 tablespoons butter until onion is soft and translucent. Stir in flour, salt and pepper. Cook for 1 minute until flour is well blended. Add sour cream and cheese. Cook, stirring constantly, over low heat until cheese melts. Spoon asparagus into a buttered shallow baking dish. Top with cheese, onion and sour cream mixture. In a small skillet heat the remaining tablespoon of butter, and when it has melted toast the Panko crumbs for about 1 minute. Bake at 400 degrees for 20 minutes. If top is not brown, place under broiler for an additional minute or two.