Build a multilevel fire in your grill. When the colas are all ignited, the flames have died down, and the temperature is hot, you are ready to cook.
Combine the cumin, coriander, chili powder, salt, and pepper in a small bowl and mix well.
Rub the steak with some of the oil, then sprinkle it with about two-thirds of the spice mixture, pressing down gently to be sure it adheres. Put the steak on the grill directly over the hottest part of the fire and sear very well for 5-8 minutes per side, then move to the cooler part of the grill and continue to cool until it is done to your liking (8-10 minutes more per side for rare). To check for doneness, poke the meat with your finger to test its firmness. If you are unsure, make a cut into the center; it should be just slightly less done than you like it. Remove from the grill, cover loosely with foil, and allow to rest for 5 minutes.
While the steak is searing, rub the leeks with the remaining oil and sprinkle with the remaining spice rub. When you move the steak to the cooler side of the grill, lay the leeks beside it and cook them, rolling around occasionally for even cooking, until they are golden brown and slightly charred (8-12 minutes). Transfer the leeks to a cutting board, halve them lengthwise, then slice crosswise into half-moons about 1/2 inches (1cm) wide. Cover to keep warm until steak is done.
Mix the mustard and the apricots together until evenly combined.
Transfer the steak to a platter (halved or sliced as you wish) and arrange the leeks, cut side up, around meat. Dab both with the apricot mustard and serve, passing additional mustard on the side.