Trim fat from steak. Score both sides of steak in a diamond pattern, making shallow diagonal cuts at 1-inch intervals. Rub pepper into steak. Place steak in a resealable plastic bag set in a shallow dish. For marinade, in a small bowl stir together Italian dressing and mustard. Pour marinade over steak; seal bag. Marinate in the refrigerator for 2 to 24 hours, turning bag occasionally. Drain steak, reserving marinade.
For a charcoal grill, grill steak on the rack of an uncovered grill directly over medium coals for 17 to 21 minutes for medium doneness (160 degrees F), turning and brushing once with marinade halfway through grilling. (For a gas grill, preheat grill. Reduce heat to medium. Place steak on grill rack over heat. Cover and grill as above.)
Peel yams or sweet potatoes; cut into 1/4-inch diagonal slices. In a medium bowl stir together peanut oil or cooking oil, salt, and, if desired, cayenne pepper. Gently toss yam slices in oil mixture. Arrange yam slices in a grill wok. For a charcoal grill, place wok on the rack of an uncovered grill directly over medium coals. Grill about 15 minutes or until yam slices are tender, turning occasionally. (For a gas grill, preheat grill. Reduce heat to medium. Place wok on grill rack over heat. Cover and grill as above.)