How to Make It
Spread the lamb on a work surface and lightly pound the thickest part of the meat to a 2-inch thickness. Put the lamb in a large resealable plastic bag. Add the lime juice, chile powder, oregano, 1 teaspoon of salt, 1/2 teaspoon of pepper and 2 tablespoons of the olive oil and turn to coat. Refrigerate for at least 8 hours or overnight.
Remove the lamb from the marinade and blot it dry with paper towels. Bring the lamb to room temperature.
Light a grill and oil the grates. Brush the lamb with olive oil and season with salt and pepper. Grill over moderate heat until lightly charred on the underside, about 10 minutes. Turn the lamb over and scatter the sage and rosemary on top. Grill the lamb, turning once more, until an instant-read thermometer inserted into the thickest part of the meat registers 135° for medium-rare or 140° for medium, 20 to 25 minutes longer. Transfer the lamb to a work surface and let rest for 15 minutes.
Meanwhile, in a large bowl, toss the tomatoes, poblanos, onion wedges and scallions with the remaining 2 tablespoons of olive oil and season with salt and pepper. Grill over moderately high heat, turning occasionally, until lightly charred and just tender, 4 to 7 minutes. Transfer the vegetables to a serving platter. Thinly slice the lamb across the grain and transfer it to the platter. Serve with the Bacon Fat Tortillas and Tomatillo Salsa.