Cook first 6 ingredients over low to medium heat in large pan. When this is smooth and bubbly, add the sour cream (must be fresh or it will curdle) and stir until mixed. Stir in Swiss cheese; once melted, fold in well-drained string beans. Transfer to casserole dish.
Sprinkle top with cornflakes. Pour melted butter on top. Bake at 350 degrees for 45 minutes.
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