1. Combine cumin, oregano, 1 2 teaspoon salt, the chili powder and garlic in a small bowl, for spice mixture; set aside.
2. Preheat broiler and arrange rack 6 inches from heat. Line a baking sheet with foil. Rinse steak and pat dry; lightly season both sides with salt and transfer to baking sheet. Broil until edges are brown and sizzling, about 6 minutes. Turn and broil 6 minutes more for medium; remove from oven. Evenly sprinkle spice mixture over top of steak and, using the back of a spoon, rub into steak. Transfer to a cutting board and let stand 10 minutes.
3. Meanwhile, combine zest, juice, vinegar, cilantro, honey and remaining 12 teaspoon salt in a medium bowl; whisk until just combined. Add oil in a slow stream, whisking constantly, until thickened and emulsified, for dressing; set aside.
4. Gently toss together mango, greens and 3 to 4 tablespoons dressing in a large serving bowl until coated. Halve, pit, peel and slice avocado. Thinly slice steak across the grain and arrange with avocado alongside salad in serving plates. Serve remaining dressing on the side.