In a medium saucepan, boil 4 cups of water. Place anchos into boiling water and cover, turn off heat. Let stand and steep for 15 minutes or until chiles are soft. Strain the chiles and reserve the chile water.
Into a blender, add the ancho chiles, serranos, garlic, cilantro, oregano, cumin, cinnamon, black pepper, soy sauce, olive oil and lime juice. Blend on medium speed until a paste forms. If this mixture is too thick, add some of the reserved chile water.
Put the flank steak and the ancho marinade into a large bowl. Thoroughly coat the beef on all sides, mixing with your hands if necessary. Cover bowl with plastic wrap and tin foil. Let beef marinate in your refrigerator for 2 to 5 days. (The longer you let it marinate, the stronger the flavors will be.).
Grill and cook the flank steak to your liking over a charcoal or mesquite fire.
When done, slice the meat thinly, cutting across the grain of the meat. Place meat on serving platter. Garnish with sprigs of fresh cilantro.