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1
Pat steak dry with paper towels; season generously with salt and pepper. Let sit at room temperature while you make the dressing or up to 1 hour.
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2
Meanwhile, whisk shallot, mustard, and vinegar in a medium bowl. Whisking constantly, gradually stream in 5 Tbsp. oil. Whisk until dressing is thick and emulsified, then whisk in 1 Tbsp. water; season with salt. Taste and add a splash of vinegar if needed.
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3
Heat a dry cast-iron skillet over medium-high. Pat steak dry again. Rub with remaining 1 Tbsp. oil. Cook until golden brown, about 3 minutes per side. Using tongs, turn steak onto fat cap and cook until browned, about 3 minutes. Turn steak back onto a flat side and cook, turning every minute or so, until medium-rare (an instant-read thermometer inserted into the thickest part should register 125u00b0), about 4 minutes longer. Transfer to a cutting board; let rest 10 minutes.
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4
Wipe out skillet and let cool 2 minutes. Set skillet over medium-high; cook corn, thyme, and butter, stirring occasionally, until corn is browned in spots, 5-7 minutes.
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5
Arrange lettuce on a platter; season with salt and pepper. Whisk dressing to reincorporate and drizzle half over lettuce. Toss cucumbers and tomatoes in a medium bowl; season with salt and pepper. Drizzle in 2 Tbsp. dressing; toss again. Spoon over lettuce. Thinly slice steak crosswise; arrange on top. Drizzle remaining dressing over and top with corn.
