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1
Divide ground beef into 8 equal balls (about 2 oz each).
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2
Heat a heavy cast-iron skillet over high heat until just smoking.
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3
Place a ball on the skillet and smash flat with a sturdy spatula. Season with salt and pepper.
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4
Cook 90 seconds until edges are crispy and dark brown.
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5
Flip, top with a slice of cheese, and cook 30 more seconds until cheese melts.
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6
Warm tortillas in a dry pan, 20 seconds per side.
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7
Mix mayo, ketchup, and relish; spread on tortillas.
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8
Top with two patties, lettuce, tomato, and onion. Serve immediately.