- 2 tablespoons hazelnut, walnut, or olive oil
- 2 tablespoons Sherry vinegar, divided
- Kosher salt, freshly ground pepper
- 2 tablespoons vegetable oil
- 1 1 1/2 pound hanger steak, center membrane removed, cut into 4 equal pieces
- 2 tablespoons unsalted butter
- 1 medium shallot, finely chopped
- 1 tablespoon finely chopped fresh thyme
- 2 teaspoons black peppercorns, coarsely chopped
- 2 teaspoons dry green peppercorns, coarsely chopped
- 1 tablespoon Dijon mustard
- Vegetable oil (for frying; about 3 1/2 cups)
- 3/4 cup buttermilk
- 2 tablespoons apple cider vinegar
- 1 1/2 cups all-purpose flour
- Kosher salt, freshly ground pepper
- 1 large onion, sliced 1/8″ thick, rings separated
- 6 cups watercress leaves with tender stems
- Special equipment: A deep-fry thermometer
- Whisk hazelnut oil and 1 tablespoon vinegar in a small bowl; season with salt and pepper. Set vinaigrette aside.
- Heat vegetable oil in a large skillet over medium-high heat. Season steak with salt and pepper and cook 6–8 minutes per side for medium-rare. Let rest 10 minutes.
- While steak rests, cook butter and shallot in same skillet over medium heat, stirring occasionally, until shallot is softened and starting to brown, about 4 minutes. Add thyme and peppercorns. Cook, stirring occasionally, until mixture is fragrant, about 2 minutes. Add remaining 1 tablespoon vinegar and 1/2 cup water and simmer until flavors meld and sauce is thick enough to coat a spoon, about 2 minutes. Remove sauce from heat and whisk in mustard; season with salt and pepper.
- Fit a medium saucepan with thermometer; pour in oil to measure 3″. Heat over medium-high heat until thermometer registers 350°F.
- Meanwhile, mix buttermilk and vinegar in a shallow bowl. Place flour in another bowl or baking dish; season with salt and pepper. Toss onion rings in flour mixture, shaking off excess, and transfer to a wire rack. Working in batches, dip in buttermilk mixture, letting excess drip back into bowl; toss again in flour.
- Working in 2 or 3 batches and maintaining temperature of oil, fry onion rings until golden brown and crisp, about 3 minutes per batch. Let drain on paper towels; season with salt.
- Toss watercress with reserved vinaigrette. Thinly slice steak against the grain. Serve steak with sauce, watercress, and onion rings.