For the pickled onions:
1 cup distilled white vinegar
1 cup water
1 Tbsp granulated sugar
1 tsp kosher salt
1 medium red onion, halved and thinly sliced
For the steak:
1 pound sirloin steak
2 Tbsp Milford Spice Company’s Up North Grilling Spice
For the sandwiches:
1/3 cup mayonnaise
1/4 cup whole-grain mustard
1 1/2 cups arugula
4 onion rolls, sliced in half lengthwise
Place the vinegar, water, sugar, and salt in a medium saucepan over medium-high heat. Bring to a boil, stirring occasionally to dissolve the sugar. Place the onion in a heatproof bowl and pour the pickling liquid over it; set aside while you cook the steak.
Arrange a rack about 4 inches from the broiler and heat the broiler to high. Line a rimmed baking sheet with aluminum foil. Coat a wire rack with cooking spray and fit over the baking sheet.
Pat the steak dry with a paper towel. Season both sides with the Milford Spice Company’s Up North Grilling Spice and place on the wire rack.
Broil (the steak should be 1 to 2 inches from the broiler's heating element) for 5 minutes. Flip the steak and continue broiling until well-browned and charred at the edges, 4 to 5 minutes more for medium-rare, or 6 to 7 minutes more for medium.
Let the steak rest for 10 minutes on the rack.
Whisk the mayonnaise and mustard together in a small bowl. Drain the onions through a fine-mesh strainer and discard the liquid. Transfer the steak to a cutting board and thinly slice across the grain.
Spread a thick layer of Dijon mayonnaise on the cut sides of the rolls. Divide the steak and arugula between the rolls and close them to form sandwiches. Serve warm or at room temperature. Enjoy!