Grease a 9x12-inch baking pan with butter. Combine Cheddar cheese and Swiss cheese in a bowl. Remove and discard bread crusts. Arrange 1/2 the bread on the bottom of the pan. Top evenly with 1/2 the ham and then 1/2 the cheese mixture. Repeat layers of bread, ham, and cheese mixture. Combine eggs, milk, onion salt, and dry mustard in a bowl. Pour over layers in the baking pan, cover, and refrigerate strata 8 hours to overnight. The next morning, preheat the oven to 375 degrees F (190 degrees C). Remove strata from the refrigerator and uncover. Combine corn flakes and melted butter in a bowl and pour evenly over strata. Bake in the preheated oven until set, 50 to 60 minutes. Remove from the oven and let strata sit for 10 minutes before serving.