TOTAL TIME: 30 MINS
1/2 small red onion, thinly sliced
3 tbsp. fresh lemon juice, divided
Kosher salt and pepper
2 tbsp. balsamic vinegar
2 tbsp. whole-grain mustard
1/2 tbsp. Dijon mustard
1 1/4 lb. hanger steak, trimmed
4 small ciabatta rolls, split
1 bunch arugula, thick stems discarded
1 tbsp. olive oil
1 oz. Parmesan cheese, shaved
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.