Step 1

Marinate the steak: Place the steak in a single layer in a 9x13-inch baking dish or in a resealable gallon plastic bag; set aside. Whisk the remaining ingredients in a small bowl until the sugar is dissolved. Pour over the steak and turn the steak to coat. Cover the baking dish or seal the bag, and refrigerate at least 1 hour or up to 4 hours.

Step 2

Cook the steak and mushrooms: Heat the oil in a large frying pan over medium-high heat until shimmering. While the oil is heating, remove the steak from the marinade and pat dry with paper towels. Add the steak pieces to the pan (it's okay to crowd the pan as long as they're in a single layer) and sear until browned and the thickest piece registers 120°F to 125°F for medium-rare steak, 3 to 4 minutes on each side. (Or cook to 130°F to 135°F for medium steak, and 140°F to 145°F for medium-well steak — add a minute or two for every 10 degrees needed.) Transfer to a serving platter and set aside in a warm place or cover loosely with aluminum foil.

Step 3

Add the mushrooms to the pan and cook, stirring occasionally and adjusting the heat if needed, until tender, about 5 minutes — they will release a lot of liquid. Add the vinegar and simmer 1 minute. Remove from the heat, add the butter, and stir until melted and the sauce in the pan is glossy. Taste and season with salt and pepper as needed. Pour over the steak, top with the parsley if desired, and serve.