1.
Combine mustard and red onions. Coat both sides of steak with the sauce, reserving a small amount. Place in shallow dish, cover with plastic wrap and refrigerate for 1 hour to marinate.
2.
Combine salt, pepper, garlic powder, coriander, and paprika to create a spice rub. Season steak liberally with mixture, about 1 teaspoon per side.
3.
Heat your gas or charcoal grill to high (Here's how to tell if it's high: Hold your hand about four inches above the cooking surface. After 2 to 3 seconds, the heat will force you to pull your hand away.)
4.
Oil the grill grate, using an oiled rag.
5.
Brush excess onion/mustard mixture off steaks and place meat on the grill. Use these cooking times to cook your steak to medium: 2 to 3 minutes per side for a 1/2-inch thick steak, 4 to 6 minutes per side for a 1-inch thick steak, 6 to 9 minutes per side for a 1 1/2 to 2-inch thick steak.
6.
Take the steak off the grill. Let it rest 2 to 3 minutes to allow the juices to reabsorb into the steak. Serve.