In a large skillet over high heat, add the oil. Season the steak with salt and pepper. When the oil is hot, put the steaks into the pan and cook until browned on both sides, 3 to 4 minutes per side.
In the sleeve of a slow cooker, add the onion and cilantro stems. Put the steak on top and sprinkle with garlic and chile powder. Add the tomatoes with their juice (set aside 1/2 cup of the tomatoes for the salsa), broth, and 2 cups water. Cover and cook on low for 6 to 8 hours, or until the steak is tender and shreds easily. Remove the meat and shred using 2 forks. (Strain and reserve the cooking liquid for Round 2 Recipe Crispy Steak Sandwiches.)
While the meat is cooking, combine the reserved 1/2 cup tomatoes with the scallions, lime juice, salt and pepper, to taste, and 3 tablespoons chopped cilantro leaves. Cover and refrigerate until ready to use.
Wrap the tortillas in a damp towel and microwave on high for about 30 seconds, or until heated through. Put 1/4 cup of the meat onto each tortilla (reserve 1 1/2 cups beef for Round 2 Recipe Crispy Steak Sandwiches.). Top with some salsa, cilantro leaves, and 1 tablespoon sour cream. (Reserve remaining 1 cup sour cream for Online Round 2 Recipe Creamy Mexican Dip with Spiced Chips.) Roll up and serve.