Step 1Combine the lime juice, chipotle, garlic, oregano and cumin in a bowl and mix to blend. Set aside.
Step 2Heat a cast-iron or other heavy skillet over high heat until it is smoking. Cut the steak into pieces large enough to fit into the skillet in one layer. Season each well on both sides with salt and pepper. Lay the pieces into the pan and cook until rare, 2 to 4 minutes on each side depending on the thickness of the meat. Transfer to a cutting board and let stand 5 minutes.
Step 3Cut the meat across the grain into thin slices. Transfer to a bowl with the juices. Add the lime juice mixture and toss to mix. Season with more salt and pepper if you like.
Step 4Just before serving, add the cilantro, green onions and avocado and toss to mix. Sprinkle with the pumpkin seeds and serve.