Stir 1 Tbsp butter into a large skillet over medium-low heat. Stir onions into melted butter.
Step 2
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Put brown sugar in after 10 minutes of cooking onions until transparent. Caramelize onions in 20–40 minutes, stirring every 5 minutes. Stir in salt and pepper.
Step 3
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Apply remaining 2 Tbsp butter to one side of all four bread slices. Thinly spread dijon on the other bread slices.
Step 4
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Place two butter-side-down bread slices in a skillet over medium-low heat. Add 2 cheese slices to each bread slice. Add caramelized onions and corned beef.
Step 5
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Put the other buttered bread slice. After a few minutes, carefully flip the sandwich. Melt cheese and toast bread.
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