Heat oil in a large skillet over medium-high heat. Add tomatillos and next 3 ingredients; sauté 7 minutes or until lightly browned.
Pulse tomatillo mixture, cilantro, and 1/4 teaspoon salt in a food processor until coarsely chopped. Transfer to a medium bowl. Set aside.
Combine skirt steak, remaining 4 ingredients, and water to cover in a large saucepan. Bring to a boil; cover, reduce heat, and simmer 1 hour and 45 minutes or until meat is tender.
Remove steak to a medium bowl, discarding onion and seasonings. When cool enough to handle, shred meat. Stir in tomatillo sauce. Serve with fresh sopes.
Tip: You may substitute flank steak for skirt steak. Cook 2 1/2 hours or until tender.