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In a saute pan, melt the butter and add the onions and apple cider vinegar. Cook on medium heat for about 7-10 minutes (add more vinegar if necessaruntil the onions are soft and brown.
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Meanwhile, mix together the horseradish and dijon.
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Spread the dijon mixture onto one slice of bread and put a layer of cheese on top.
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Add 3 ounces of corned beef onto each sandwich, then the caramelized onions on top.
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Grill on a panini/foreman grill or on a buttered saute pan until the cheese has melted and the bread is golden brown.