Cut poblanos in half lengthwise; discard seeds and membranes. Place poblanos, skin sides down, on grill rack. Grill 15 minutes or until blackened. Place in a zip-top plastic bag; seal. Let stand 15 minutes. Peel and dice. Stir in cilantro, juice, 1/4 teaspoon salt, and 1/8 teaspoon black pepper.
Sprinkle steak with 1/4 teaspoon salt and 1/8 teaspoon black pepper. Place on grill rack; grill 8 minutes on each side or until thermometer registers 145° (medium-rare) or until desired degree of doneness. Remove from grill; let stand 5 minutes. Cut steak diagonally across the grain into thin slices. Serve with relish.