- 1 teaspoon vegetable oil
- 2 New York strip (shell) steaks (8 ounces each)
Coarse salt and ground pepper
- 1 medium yellow onion, thinly sliced
- 8 ounces white button mushrooms, thinly sliced
- 1 tablespoon red-wine vinegar
- 1 bag (10 ounces) baby spinach
- 4 teaspoons grainy or Dijon mustard
- 8 slices crusty bread, toasted
- In a large skillet, heat oil over medium-high. Season both sides of steaks with salt and pepper. Cook steaks until medium-rare, about 5 minutes per side, flipping once. Transfer to a plate; tent loosely with foil to keep warm.
- Add onion to pan; season with salt and pepper. Cook until onion begins to soften, about 4 minutes. Add mushrooms; season with salt and pepper. Cook until mushrooms brown, about 6 minutes. Add vinegar and any accumulated juices from steaks; cook until evaporated, about 30 seconds. Transfer mushroom mixture to a medium bowl. Add spinach to pan; season with salt and pepper. Cook, stirring, until wilted, about 3 minutes.
- Thinly slice steaks against the grain. Spread mustard on 4 slices bread. Dividing evenly, top with sliced steak, mushroom mixture, and spinach. Top with remaining bread. Serve immediately.