2 pounds hanger steak (2 steaks, veins removed to make 4 pieces) or 2 pounds skirt steak cut into 4 pieces, pounded lightly if necessary for uniform thickness
Salt and pepper
1 bunch flat-leaf parsley, leaves separated from stems
2 large cloves garlic, crushed
1 lemon, zested (about 1 1/2 tsp.) and juiced (about 1/4 cup)
About 1/2 cup freshly grated horseradish root or 1/3 cup prepared horseradish (I like Gold’s)
About 1/4 cup EVOO
2 tablespoons olive oil, plus more for drizzling
2 tablespoons butter
4 medium Portobello mushroom caps, sliced
1 onion, thinly sliced About 1/4 cup sherry About 2 tablespoons
Worcestershire sauce 2 bunches watercress or upland cress, stemmed
Season the steaks with salt and pepper. Preheat a cast-iron skillet or griddle over medium-high.
On a cutting board, finely chop the parsley leaves and the garlic. Add the lemon zest and season with salt; finely chop together and transfer to a small bowl. Stir the lemon juice, horseradish and EVOO into the gremolata.
In a large skillet, heat 2 tablespoons olive oil, two turns of the pan, over medium-high. Add the butter and let it melt into the oil. Add the mushrooms and cook, stirring occasionally, until browned, 5 to 6 minutes. Add the onion and season. Cook, stirring occasionally, until the onion is just tender, 8 to 10 minutes. Stir in the sherry, then the Worcestershire; remove from the heat.
Drizzle the steaks with oil; add to the cast-iron skillet and cook, turning occasionally, until medium-rare, 8 to 10 minutes for hanger steaks and 7 to 8 minutes for skirt steaks. Let rest a few minutes, then slice against the grain. Top with the gremolata and serve the mushroom mixture and the watercress alongside.