Make the rub: using a spice grinder or mortar and pestle, crush the black peppercorns and mustard seeds.
In a small bowl, mix well with the remaining rub ingredients.
Trim steaks of excess fat; press rub into both sides of the steaks; lightly brush or spray with olive oil.
Let stand at room temperature for 20-30 minutes before grilling.
Make the sauce: in a saucepan, whisk together the sauce ingredients with ½ cup water; simmer over low heat for about 5 minutes; set aside.
Sear the steaks over direct high heat for 10 minutes, turning once halfway through searing time.
Continue grilling over indirect medium heat until the internal temperature reaches 135° for medium-rare, 3-5 minutes, turning once halfway through grilling time (or cook until desired degree of doneness).
Remove the steaks from the grill and let rest for 5-10 minutes; cut the steaks across the grain into ¼-inch slices and serve warm with the sauce.