Heat broiler. Season the steak with ½ teaspoon each salt and pepper and place on a rimmed broilerproof baking sheet.
Broil the steak to desired doneness, 3 to 4 minutes per side for medium-rare. Let rest at least 5 minutes before slicing.
Warm the tortillas according to the package directions.
Meanwhile, in a medium bowl, combine the onion, cilantro, pepitas, oil, lime juice, and ¼ teaspoon each salt and pepper.
Serve the steak with the relish and tortillas.