Pat the steak dry and cut it against the grain into 2 or more pieces to fit in a large skillet. (You’ll clearly see the long lines running down the meat; you want to slice perpendicular to those lines.)
Put 2 tablespoons butter in the skillet over low heat. When the butter melts, add the steak, sprinkle the top with salt, turn it, and salt the other side. Cook, undisturbed, until it begins to firm up and lose its pinkness on the edges, 5 to 10 minutes, depending on the thickness; turn and cook the other side until it’s no longer pink on that side, but still quite red at the center, 1 to 5 minutes more, depending on the thickness.
Raise the heat to high and sear the steak until it’s one step before your desired doneness, probably less than 1 minute on each side. Transfer the steak to a platter and add the onions and remaining tablespoon butter to the pan; sprinkle with salt. Cook, stirring a few times, until the onions soften and char in places without burning, 3 to 5 minutes.
Pour any accumulated juices from the steak into the pan with the onions along with the lemon juice, 2 tablespoons water, and lots of pepper. Scrape up any browned bits from the bottom of the pan and cook, stirring constantly, until the sauce is thick and glossy, just a minute or 2. Slice the steak against the grain as thin as you can, and serve it topped with the onions and pan juices or return the slices to the skillet to toss in the sauce and garnish with a sprinkle of salt and pepper.