3 large Yukon Gold potatoes (1 1/4 pounds), peeled and sliced 1/4 inch thick
1 large Vidalia onion, halved lengthwise and very thinly sliced crosswise
Kosher salt and freshly ground pepper
2 tablespoons red wine vinegar
Four 7-ounce sirloin steaks (1 inch thick)
2 tablespoons unsalted butter
4 large shallots, very thinly sliced
1/2 cup dry red wine
2 tablespoons finely chopped flat-leaf parsley
In a very large nonstick skillet, heat 2 tablespoons of the olive oil until shimmering. Add the potatoes in a slightly overlapping layer and cook over moderately high heat, stirring occasionally, until golden and nearly cooked through, about 8 minutes. Add the onion, season with salt and pepper and cook, stirring occasionally, until tender, about 5 minutes longer. Add 1/2 tablespoon of the vinegar and gently toss to combine. Cover loosely and keep warm.
Meanwhile, in another large skillet, heat the remaining 1 tablespoon of olive oil until shimmering. Season the steaks with salt and pepper and cook them over moderately high heat, turning occasionally, until browned and crusty, 8 to 10 minutes for medium rare. Transfer the steaks to a serving platter and keep warm.
Add 1 tablespoon of the butter and the shallots to the skillet and cook over moderate heat until softened, about 5 minutes. Add the remaining 1 1/2 tablespoons of vinegar and cook until evaporated, about 30 seconds. Add the wine and cook until reduced by two-thirds, about 2 minutes. Remove from the heat and swirl in the remaining 1 tablespoon of butter. Add the parsley and any accumulated meat juices, season the sauce with salt and pepper and spoon over the meat. Serve the steaks at once with the potatoes.