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Preheat a grill pan over medium-high heat.
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Brush both sides of the steak with olive oil and sprinkle with salt and pepper.
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Grill the steak for about 4-5 minutes per side, or until it reaches your desired level of doneness.
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Remove the steak from the pan and let it rest for a few minutes.
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While the steak rests, add sliced onions and bell peppers to the same pan and cook for 2-3 minutes or until they are tender and slightly charred.
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Toast the bread slices until lightly browned.
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Slice the steak thinly against the grain.
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To assemble the sandwich, place a slice of bread on a plate and layer with sliced steak, sautéed onions and bell peppers, and any optional toppings such as sliced cheese, lettuce, mayo or mustard.
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Top with another slice of bread and repeat for the other sandwich.
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Serve hot and enjoy!