1. Preheat a grill to high.
2. Crush garlic clove, sprinkle with 1/4 teaspoon salt, and mash into a coarse paste using flat side of a large knife. Mix garlic paste together with mango, scallions, habanero, lime juice, 1 tablespoon olive oil, and cilantro. Set aside.
3. Pat steak dry and trim and if needed, cut in half to fit grill. Combine remaining 3 tablespoons olive oil, allspice, 1 teaspoon salt, and 1/2 teaspoon pepper together in a small bowl and rub on both sides of steak. Grill until slightly charred, crisp, and medium-rare, 3 to 4 minutes per side. Remove to a cutting board and let rest for 5 minutes. Slice meat into about 3-inch long sections; thinly slice each section.
4. Place sliced steak on a serving platter with salsa, romaine leaves, and cucumber slices.
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