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.)