Special equipment: a meat mallet; butcher's twine
Prepare an outdoor grill for direct and indirect grilling. (If your grill has a thermometer, aim to keep the temperature at about 350 degrees F throughout grilling.) Cook the poblano and bell peppers on the grill, turning occasionally, until completely charred on the outside, about 15 minutes. Transfer to a bowl and cover with a plate to steam for about 10 minutes.
Meanwhile, grill the onions until golden and soft, about 10 minutes. Transfer to a plate. Peel the poblano and bell peppers and remove and discard the stems and skins. Cut the flesh into 1/2-inch strips and set aside. (Keep the poblano strips separate from bell pepper strips.)
Set the steak on a cutting board. Using a long thin knife and starting at the thin, narrow end of the steak, slice it in half horizontally leaving 2 inches of the meat uncut at the wide end of the steak so the pieces stay attached. Open it up like a book so you have one long piece. Pound the steak with a meat mallet to an even thickness of about 1/4 inch.
Arrange the steak flat on a work surface with a long side facing you. Combine the cilantro, tomato paste, oil, cumin, oregano, scallions and 2 teaspoons salt in a bowl. Spread the paste all over the top of the steak. Working in three batches, arrange vertical stripes of the poblanos, bell peppers, onions, and olives on the steak until the steak is covered, leaving 2 inches empty at each end.
Roll up the steak like a jelly roll, starting at one of the short ends. Tie the steak closed around the circumference with butcher's twine at 2-inch intervals. Tie one more piece of twine lengthwise around the steak.
Put the steak on the direct heat side of the grill and cook, turning, to brown all 4 sides, about 2 minutes per side. Move the steak to the indirect heat side of the grill, cover and cook, turning occasionally, until the center of the steak registers 115 degrees F on an instant-read thermometer, 25 to 30 minutes for a steak that is well-done on the outside and medium-rare in the center. (Test frequently; the time will vary based on the temperature of your grill.) Let the steak rest about 10 minutes.
Untie the steak and cut it into 6 thick slices.
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