Open-Face Steak Sandwich with Parmesan Dressing Recipe | Say Mmm

Ingredients

• 1 1/2 pounds flank steak
• Kosher salt, freshly ground pepper
• 1 tablespoon vegetable oil
• 1/2 small red onion, thinly sliced
• 1/4 cup whole grain mustard
• 3 tablespoons fresh lemon juice
• 1/2 cup olive oil, plus more for bread
• 1 1/2 ounces Parmesan, finely grated
• 1 ciabatta loaf, halved lengthwise
• 6 cups mature arugula leaves with tender stems