In a large pan, melt the butter and add the onion and apple slices. Let them cook on low heat, uncovered, stirring occasionally for about 10 minutes.
Season with salt and pepper, then cover and cook another 10 minutes or so, or until the onions are soft.
Add the franks, and roll them around in the mixture. Cover the pan again, and cook for an additional 10 minutes, or until the franks have slightly browned.
Toast the hot dog buns.
On each bun, put some of the mixture, and lay a frank on top.
Top with mustard, ketchup, and relish if desired.
Notes
Use good beef franks that will marry with the flavor of the apples and onions. If you are watching your carbohydrates, omit the bun.
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