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 tbsp. 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.