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Heat a skillet over medium low heat. Add oil and onion. Cook onion for about 20 minutes or until onion is tender and browned. Stir frequently. Remove onion from heat.
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Saute the chopped kale until just wilted. Season with salt and pepper.
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Spread butter on the outside of the slices.
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Stack two slices of cheese, half of the onions, half of kale and 2 slices of tomato on the unbuttered side of the bread. Finish off with the other buttered slice of bread.
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Heat the skillet over medium heat and place each sandwich on the skillet. Cook until the bread is toasted and cheese has melted and flip over and toast the other side. Do the same for the other sandwich.
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Serve immediately.