Grilled Picanha Sandwich with Chimichurri

Trim and remove any silver skin from picanha; trim fat cap to about 1/2-inch thickness. Score fat cap horizontally and vertically at 1/2-inch intervals, creating a crosshatch pattern and making sure not to pierce flesh. Slice picanha with the grain into 3 (about 3-inch-thick) steaks; transfer steaks to a rimmed baking sheet. Stir together granulated garlic, cumin, and 2 teaspoons salt in a small bowl. Season steaks on all sides with garlic-salt mixture. Set aside at room temperature for 1 hour.

Preheat grill to medium (350°F to 400°F). Lightly grease grill grates with canola oil; place steaks on grates, fat cap side down. Grill, uncovered, until fat begins to render and is golden brown, 6 to 8 minutes, moving steaks around as needed to prevent flare-ups. Turn steaks, and continue grilling, turning occasionally, until browned on all sides and a thermometer inserted into thickest portion of steaks registers 120°F (for medium-rare), 15 to 20 minutes, or until desired degree of doneness. Transfer steaks to a cutting board; let rest for 15 minutes. Thinly slice steaks against the grain.

Meanwhile, brush cut sides of ciabatta evenly with 1 tablespoon olive oil. Place ciabatta halves, cut sides down, on oiled grates. Grill, uncovered, until toasted, 1 to 2 minutes; set aside. Mash avocados with lemon juice, 1/2 teaspoon salt, and remaining 1 tablespoon olive oil until creamy but still slightly chunky.

Arrange sliced steak evenly over bottom ciabatta half, reserving any excess steak for another use. Spoon chimichurri evenly over steak. Layer sliced tomatoes over top, and sprinkle evenly with remaining 1/4 teaspoon salt. Spread avocado mixture over tomatoes. Spread mayonnaise evenly on top ciabatta half, and close sandwich; cut crosswise into quarters.