How to Make It
In a medium bowl, combine the za'atar with the oil and a generous pinch of salt. Add the pork, turn to coat and let stand at room temperature for 30 minutes.
Meanwhile, roast the poblanos directly over a gas flame or under a broiler, turning frequently, until blackened; transfer to a bowl, cover with plastic wrap and let cool slightly. Peel and seed the chiles; cut into thin strips.
In a large nonstick skillet, heat 1 tablespoon of the oil from the pork marinade. Add the pork and cook over high heat, turning once, until white throughout and lightly browned, 5 minutes. Transfer the pork to a platter.
Add any remaining marinade oil to the skillet. Add the onion and poblano and cook over moderately high heat, stirring, until the onion is softened, 5 minutes. Add the tomatoes and toss. Add 1/2 cup of water and cook until the tomatoes are just softened, 4 minutes. Spoon the sauce over the pork and serve.