1. Combine 1 teaspoon rind, 2 teaspoons oil, 1/4 teaspoon salt, black pepper, and garlic in a small bowl.
2. Score a diamond pattern on both sides of steak. Rub both sides of steak with oil mixture. Cover and refrigerate 2 hours.
3. Prepare grill to medium-high heat.
4. Place steak on a grill rack coated with cooking spray; grill 6 minutes on each side or until desired degree of doneness. Let stand 5 minutes. Cut steak diagonally across grain into thin slices.
5. Combine remaining 1 teaspoon rind, remaining 2 teaspoons oil, remaining 1/4 teaspoon salt, diced avocado, tomato, red onion, lime juice, and jalapeño pepper in a small bowl. Serve sliced steak with the relish and lime wedges.
Wine note: When it comes to red meat, fattier cuts benefit from big red wines, whose drying tannins help to balance the food. With a leaner cut and a summery preparation like Grilled Flank Steak with Avocado Relish, opt for a supple California merlot, like Clos du Bois Merlot North Coast 2005 ($15). This wine has soft tannins and juicy plum and black cherry flavors that meld with the fruity combination of avocado and tomato. —Jeffery Lindenmuth