Prepare the steaks: Pulse the coriander seeds, peppercorns and mustard seeds in a spice grinder until coarsely ground. Transfer to a small bowl and add the brown sugar, paprika, garlic powder and allspice; mix until evenly combined. Season the steaks all over with salt and pepper, then rub with the vegetable oil. Rub the steaks on both sides with the spice mixture, pressing to adhere. Let sit at room temperature, 30 minutes.
Meanwhile, preheat a grill to medium high. Lightly oil the grates.
Make the mustard butter: Stir the butter in a small bowl until smooth. Mix in the mustard, parsley, chives, 1/4 teaspoon salt and a few grinds of pepper.
Grill the steaks until well marked, 4 to 5 minutes. Rotate the steaks 90 degrees, moving them around as needed to prevent flare-ups, and cook until crosshatch grill marks form, 4 to 5 more minutes. Flip the steaks and cook 4 to 5 minutes, then rotate 90 degrees and cook 4 to 5 more minutes, or until a thermometer inserted sideways into the center of the steaks registers 125 degrees F for medium rare. Transfer the steaks to a cutting board and let rest at least 5 minutes. Serve with the mustard butter.