small red onion, thinly sliced
fresh lemon juice, divided
Kosher salt and pepper
1 1/4 lb.
hanger steak, trimmed
small ciabatta rolls, split
bunch arugula, thick stems discarded
Parmesan cheese, shaved
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Heat grill to medium-high. In a small bowl, toss onion with 2 tablespoons lemon juice and a pinch each of salt and pepper. Let sit, tossing occasionally, until ready to use.
In a second bowl, whisk together vinegar and mustards. Season steak with 1/2 teaspoon each salt and pepper and grill to desired doneness, basting with vinegar mixture during the last 3 minutes of cooking, 3 to 4 minutes per side for medium depending on thickness of steak. Transfer to a cutting board and let rest at least 5 minutes before slicing.
Grill rolls until lightly charred and toasted, 1 to 2 minutes per side. In a bowl, toss arugula with oil, remaining tablespoon of lemon juice and a pinch each of salt and pepper, then toss with Parmesan. Arrange steak and arugula salad on top of rolls, then top with marinated onions, leaving any juices behind in bowl.